Multi-beam scanning apparatus

ABSTRACT

Disclosed in a multi-beam optical scanning apparatus which includes a light source unit including a plurality of radiation points disposed with being spaced from each other in a main scanning direction, and a deflecting unit for deflecting a plurality of light beams radiated from the light source unit toward a surface to be scanned. Where a first radiation point is a radiation point for radiating the light beam, out of the plurality of light beams emitted from the light source unit, which reaches the farthest location from a center of a deflecting facet of the deflecting unit in the main scanning direction, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by the deflecting unit, and which exists on an upstream side in a rotational direction of deflecting unit relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, the second radiation point is excited in the first place in the upstream-side external angular range.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a multi-beam scanning apparatus,and particularly to a scanning apparatus which is suitably applicable toimage forming apparatuses, such as laser beam printers, digital copyingmachines, and multi-function printers that employ theelectrophotographic process, and in which a light beam from a lightsource unit is reflectively deflected by a polygon mirror serving as adeflecting unit, transmitted through a scanning optical system, andscanned on a surface to be scanned (a scanned surface) to record imageinformation. More particularly, the present invention relates to amulti-beam scanning apparatus in which plural light beams aresimultaneously scanned to achieve operation with high speed and fineprecision, and a deflecting unit, such as a polygon mirror, iseffectively used to downsize the overall apparatus.

[0003] 2. Related Background Art

[0004]FIG. 21 schematically illustrates a main portion of a conventionalmulti-beam scanning apparatus.

[0005] In FIG. 21, two beams emitted from a light source unit 91 andoptically modulated according to image information are converted intoapproximately parallel light beams by a collimator lens 92, and areincident on a cylindrical lens 93. Each light beam incident on thecylindrical lens 93 emerges therefrom without any change in a mainscanning section, and passes through an aperture stop 94. The light beamis partially intercepted by the aperture stop 94. With respect to a subscanning section, each beam is converged by the cylindrical lens 93,passes through the aperture stop 94 with a portion of the light beambeing intercepted, and is imaged on a deflecting facet 95 a of adeflecting unit 95 as an approximately linear image (a linear imageextending in a main scanning direction). Each light beam reflectivelydeflected by the deflecting facet 95 a of the deflecting unit 95 isimaged on a photosensitive drum surface 97 in the form of a spot by ascanning optical system (a scanning lens) 96. The photosensitive drumsurface 97 is scanned with the imaged spot moving at a uniform speed ina direction of an arrow B (the main scanning direction) when thedeflecting unit 95 is rotated in a direction of an arrow A. Accordingly,two scanning lines are simultaneously formed on the photosensitive drumsurface 97 serving as a recording material such that image recording canbe executed.

[0006] At the time of the above-discussed operation, a portion (a BDlight beam) of each light beam reflectively deflected by the deflectingunit 95 is guided to a light detecting device (a BD sensor) 99 of asynchronous detecting unit through a folding mirror (a BD mirror) 98 ofthe synchronous detecting unit by the scanning optical system 96, sothat timing of a scanning start position on the photosensitive drumsurface 97 can be adjusted prior to the scanning of the photosensitivedrum surface 97 with the light spot. For each BD light beam, the timingof the scanning start position for image recording on the photosensitivedrum surface 97 is adjusted, using a synchronous signal (a BD signal)obtained by detecting an output signal from the BD sensor 99. In FIG.21, only a principal ray of each light beam is illustrated for theconvenience of simplicity and easy understanding.

[0007] In such a multi-beam scanning apparatus, in order to obtain ahigh-quality image, the radiation amount of each light beam is adjusted(auto-power control (APC)) such that the amount of light projected onthe photosensitive drum surface can be stably made constant, prior tothe start of writing scanning lines.

[0008] Further, there has been proposed a multi-beam scanning apparatuswhich includes a light-amount monitor for performing synchronousdetection (BD detection) and adjustment (APC) of the light amounts ofplural light beams emitted from a light source unit, and in which thesynchronous detection and the light-amount adjustment of the plurallight beams are sequentially executed.

[0009] Those conventional multi-beam scanning apparatuses, however, donot mention the light radiation order of the light source unit by anymeans. Accordingly, the deflecting unit (the polygon mirror) thereincannot be effectively used, and hence there is a need to use a largedeflecting unit.

[0010] Further, in a case where ends of the deflecting facet in thedeflecting unit are chamfered, a light beam incident on the chamferedportion is likely to reach the photosensitive drum surface, leading tooccurrence of the problem of ghost.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a multi-beamscanning apparatus which is capable of being downsized. Further, it isanother object of the present invention to provide a multi-beam scanningapparatus which can eliminate the ghost light, and is capable of alwaysachieving a preferable image.

[0012] According to one aspect of the present invention, there isprovided a multi-beam optical scanning apparatus which includes a lightsource unit having a plurality of radiation points disposed with beingspaced from each other in a main scanning direction, and a deflectingunit for deflecting a plurality of light beams radiated from theplurality of radiation points toward a surface to be scanned. In themulti-beam optical scanning apparatus, where a first radiation point isa radiation point for radiating the light beam, out of the plurality oflight beams emitted from the plurality of radiation points, whichreaches the farthest location from a center of a deflecting facet of thedeflecting unit in the main scanning direction, a second radiation pointis a radiation point for radiating another light beam, and anupstream-side external angular range is a range which lies in an angularrange over which the light beam can be deflected by the deflecting unit,and which exists on an upstream side in a rotational direction of thedeflecting unit relative to an effective scanning angular range at thetime when the light beam is deflected toward an effective scanning rangeon the surface to be scanned, control is performed such that the lightbeam from the second radiation point can be radiated prior to the lightbeam from the first radiation point in the upstream-side externalangular range.

[0013] In that multi-beam optical scanning apparatus, the light beam ofthe radiation point for radiating the light beam reaching a locationnearest a center of the deflecting facet of the deflecting unit can beradiated in the first place, out of the plurality of light beamsradiated by the light source unit. In this case, the light beam of theradiation point for radiating the light beam reaching a location nearerthe center of the deflecting facet of the deflecting unit can beradiated in the order from the nearest location, out of the plurality oflight beams radiated by the light source unit.

[0014] According to another aspect of the present invention, there isprovided a multi-beam optical scanning apparatus which includes a lightsource unit having a plurality of radiation points disposed with beingspaced from each other in a main scanning direction, and a deflectingunit for deflecting a plurality of light beams radiated from theplurality of radiation points toward a surface to be scanned, and inwhich the plurality of light beams radiated from the plurality ofradiation points intersect each other M times (M=2n+1; n is an integer)between the light source unit and the deflecting unit. In the multi-beamoptical scanning apparatus, where a first radiation point is a radiationpoint disposed on a most upstream side in a rotational direction of thedeflecting unit, out of the plurality of radiation points, a secondradiation point is a radiation point for radiating another light beam,and an upstream-side external angular range is a range which lies in anangular range over which the light beam can be deflected by thedeflecting unit, and which exists on the upstream side in the rotationaldirection of the deflecting unit relative to an effective scanningangular range at the time when the light beam is deflected toward aneffective scanning range on the surface to be scanned, control isperformed such that the light beam from the second radiation point canbe radiated prior to the light beam from the first radiation point inthe upstream-side external angular range.

[0015] In that multi-beam optical scanning apparatus, the light beam ofthe radiation point disposed on a most downstream side in the rotationaldirection of the deflecting unit can be radiated in the first place. Inthis case, the light beam of the radiation point disposed on the moredownstream side in the rotational direction of the deflecting unit canbe radiated in the order from the most downstream side.

[0016] Further, in that multi-beam optical scanning apparatus, theradiation amount of the light beam can be adjusted by radiating thelight beam from the radiation point of the light source unit in theupstream-side external angular range prior to the effective scanningrange on the surface to be scanned. That multi-beam optical scanningapparatus can further include a scanning optical system for formingimages of the plurality of light beams deflected by the deflecting uniton the surface to be scanned, and a synchronous detecting unit fordetecting writing start timings on the surface to be scanned byreceiving the plurality of light beams deflected by the deflecting unit,and in this apparatus, synchronous detection is performed by radiatingthe light beam directed to the synchronous detecting unit from theradiation point of the light source unit in the upstream-side externalangular range prior to the effective scanning range on the surface to bescanned.

[0017] Further, in that multi-beam optical scanning apparatus, achamfered portion can be formed at an edge of a deflecting facet of thedeflecting unit.

[0018] Furthermore, in that multi-beam optical scanning apparatus, wherea third radiation point is another radiation point other than the firstradiation point disposed on the most upstream side in the rotationaldirection of the deflecting unit, the light beam of the third radiationpoint can be radiated in the first place in a downstream-side externalangular range subsequent to the effective scanning range on the surfaceto be scanned.

[0019] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having a plurality of radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting a plurality of light beams radiated fromthe plurality of radiation points toward a surface to be scanned, and inwhich the plurality of light beams radiated from the plurality ofradiation points intersect each other N times (N=2n; n is an integer)between the light source unit and the deflecting unit. In the multi-beamoptical scanning apparatus, where a first radiation point is a radiationpoint disposed on a most downstream side in a rotational direction ofthe deflecting unit, out of the plurality of radiation points, a secondradiation point is a radiation point for radiating another light beam,and an upstream-side external angular range is a range which lies in anangular range over which the light beam can be deflected by thedeflecting unit, and which exists on the upstream side in the rotationaldirection of the deflecting unit relative to an effective scanningangular range at the time when the light beam is deflected toward aneffective scanning range on the surface to be scanned, control isperformed such that the light beam from the second radiation point canbe radiated prior to the light beam from the first radiation point inthe upstream-side external angular range.

[0020] In that multi-beam optical scanning apparatus, the light beam ofthe radiation point disposed on the most upstream side in the rotationaldirection of the deflecting unit can be radiated in the first place. Inthis case, the light beam of the radiation point disposed on the moreupstream side in the rotational direction of the deflecting unit can beradiated in the order from the most upstream side.

[0021] In that multi-beam optical scanning apparatus, the radiationamount of the light beam can be adjusted by radiating the light beamfrom the radiation point of the light source unit in the upstream-sideexternal angular range prior to the effective scanning range on thesurface to be scanned.

[0022] Further, that multi-beam optical scanning apparatus can furtherinclude a scanning optical system for forming images of the plurality oflight beams deflected by the deflecting unit on the surface to bescanned, and a synchronous detecting unit for detecting writing starttimings on the surface to be scanned by receiving the plurality of lightbeams deflected by the deflecting unit, and in this apparatus,synchronous detection is performed by radiating the light beam directedto the synchronous detecting unit from the radiation point of the lightsource unit in the upstream-side external angular range prior to theeffective scanning range on the surface to be scanned.

[0023] Further, in that multi-beam optical scanning apparatus, achamfered portion can be formed at an edge of a deflecting facet of thedeflecting unit.

[0024] Furthermore, in that multi-beam optical scanning apparatus, wherea third radiation point is another radiation point other than the firstradiation point disposed on the most downstream side in the rotationaldirection of the deflecting unit, the light beam of the third radiationpoint can be radiated in the first place in a downstream-side externalangular range subsequent to the effective scanning range on the surfaceto be scanned.

[0025] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having a plurality of radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting a plurality of light beams radiated fromthe plurality of radiation points toward a surface to be scanned. In themulti-beam optical scanning apparatus, the light beam of the radiationpoint for radiating the light beam firstly incident on a deflectingfacet of the deflecting unit in the main scanning direction is radiatedprior to the light beam from the other radiation point.

[0026] That multi-beam optical scanning apparatus can further include ascanning optical system for forming images of the plurality of lightbeams deflected by the deflecting unit on the surface to be scanned, anda synchronous detecting unit for detecting writing start timings on thesurface to be scanned by receiving the plurality of light beamsdeflected by the deflecting unit, and in this apparatus, synchronousdetection is performed by radiating the light beam directed to thesynchronous detecting unit from the radiation point of the light sourceunit in the upstream-side external angular range prior to the effectivescanning range on the surface to be scanned.

[0027] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having at least three radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting at least three light beams radiated fromthe at least three radiation points toward a surface to be scanned. Inthe multi-beam optical scanning apparatus, where a first radiation pointis a radiation point for radiating the light beam, out of the at leastthree light beams emitted from the at least three radiation points,which reaches the farthest location from a center of a deflecting facetof the deflecting unit in the main scanning direction, a secondradiation point is a radiation point for radiating another light beam,and an upstream-side external angular range is a range which lies in anangular range over which the light beam can be deflected by thedeflecting unit, and which exists on an upstream side in a rotationaldirection of the deflecting unit relative to an effective scanningangular range at the time when the light beam is deflected toward aneffective scanning range on the surface to be scanned, control isperformed such that the light beam from the second radiation point canbe radiated prior to the light beam from the first radiation point inthe upstream-side external angular range.

[0028] In that multi-beam optical scanning apparatus, the light beam ofthe radiation point for radiating the light beam reaching a locationnearest a center of the deflecting facet of the deflecting unit can beradiated in the first place, out of the at least three light beamsradiated by the light source unit.

[0029] Further, in that multi-beam optical scanning apparatus, the lightbeam of the radiation point for radiating the light beam reaching alocation nearer the center of the deflecting facet of the deflectingunit can be radiated in the order from the nearest location, out of theat least three light beams radiated by the light source unit.

[0030] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having at least three radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting at least three light beams radiated fromthe at least three radiation points toward a surface to be scanned, andin which the at least three light beams radiated from the at least threeradiation points intersect each other M times (M=2n+1; n is an integer)between the light source unit and the deflecting unit. In the multi-beamoptical scanning apparatus, where a first radiation point is a radiationpoint disposed on a most upstream side in a rotational direction of thedeflecting unit, out of the at least three radiation points, a secondradiation point is a radiation point for radiating another light beam,and an upstream-side external angular range is a range which lies in anangular range over which the light beam can be deflected by thedeflecting unit, and which exists on the upstream side in the rotationaldirection of the deflecting unit relative to an effective scanningangular range at the time when the light beam is deflected toward aneffective scanning range on the surface to be scanned, control isperformed such that the light beam from the second radiation point canbe radiated prior to the light beam from the first radiation point inthe upstream-side external angular range.

[0031] In that multi-beam optical scanning apparatus, the light beam ofthe radiation point disposed on a most downstream side in the rotationaldirection of the deflecting unit can be radiated in the first place.

[0032] Further, in that multi-beam optical scanning apparatus, the lightbeam of the radiation point disposed on the more downstream side in therotational direction of the deflecting unit can be radiated in the orderfrom the most downstream side.

[0033] Further, in that multi-beam optical scanning apparatus, theradiation amount of the light beam can be adjusted by radiating thelight beam from the radiation point of the light source unit in theupstream-side external angular range prior to the effective scanningrange on the surface to be scanned.

[0034] Further, that multi-beam optical scanning apparatus can furtherinclude a scanning optical system for forming images of the at leastlight beams deflected by the deflecting unit on the surface to bescanned; and synchronous detecting unit for detecting writing starttimings on the surface to be scanned by receiving the at least threelight beams deflected by the deflecting unit, and in this apparatus,synchronous detection is performed by radiating the light beam directedto the synchronous detecting unit from the radiation point of the lightsource unit in the upstream-side external angular range prior to theeffective scanning range on the surface to be scanned.

[0035] Further, in that multi-beam optical scanning apparatus, achamfered portion can be formed at an edge of a deflecting facet of thedeflecting unit.

[0036] Further, in that multi-beam optical scanning apparatus, where athird radiation point is another radiation point other than the firstradiation point disposed on the most upstream side in the rotationaldirection of the deflecting unit, the light beam of the third radiationpoint can be radiated in the first place in a downstream-side externalangular range subsequent to the effective scanning range on the surfaceto be scanned.

[0037] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having at least three radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting at least three light beams radiated fromthe at least three radiation points toward a surface to be scanned, andin which the at least three light beams radiated from the at least threeradiation points intersecting each other N times (N=2n; n is an integer)between the light source unit and the deflecting unit. In the multi-beamoptical scanning apparatus, where a first radiation point is a radiationpoint disposed on a most downstream side in a rotational direction ofthe deflecting unit, out of the at least three radiation points, asecond radiation point is a radiation point for radiating another lightbeam, and an upstream-side external angular range is a range which liesin an angular range over which the light beam can be deflected by thedeflecting unit, and which exists on the upstream side in the rotationaldirection of the deflecting unit relative to an effective scanningangular range at the time when the light beam is deflected toward aneffective scanning range on the surface to be scanned, control isperformed such that the light beam from the second radiation point canbe radiated prior to the light beam from the first radiation point inthe upstream-side external angular range.

[0038] In that multi-beam optical scanning apparatus, the light beam ofthe radiation point disposed on the most upstream side in the rotationaldirection of the deflecting unit can be radiated in the first place.

[0039] Further, in that multi-beam optical scanning apparatus, the lightbeam of the radiation point disposed on the more upstream side in therotational direction of the deflecting unit can be radiated in the orderfrom the most upstream side.

[0040] Further, in that multi-beam optical scanning apparatus, theradiation amount of the light beam can be adjusted by radiating thelight beam from the radiation point of the light source unit in theupstream-side external angular range prior to the effective scanningrange on the surface to be scanned.

[0041] Further, that multi-beam optical scanning apparatus can furtherinclude a scanning optical system for forming images of the at leastthree light beams deflected by the deflecting unit on the surface to bescanned, and synchronous detecting unit for detecting writing starttimings on the surface to be scanned by receiving the at least threelight beams deflected by the deflecting unit, and in this apparatus,synchronous detection is performed by radiating the light beam directedto the synchronous detecting unit from the radiation point of the lightsource unit in the upstream-side external angular range prior to theeffective scanning range on the surface to be scanned.

[0042] Further, in that multi-beam optical scanning apparatus, achamfered portion can be formed at an edge of a deflecting facet of thedeflecting unit.

[0043] Further, in that multi-beam optical scanning apparatus, where athird radiation point is another radiation point other than the firstradiation point disposed on the most downstream side in the rotationaldirection of the deflecting unit, the light beam of the third radiationpoint can be radiated in the first place in a downstream-side externalangular range subsequent to the effective scanning range on the surfaceto be scanned.

[0044] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having at least three radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting at least three light beams radiated fromthe at least three radiation points toward a surface to be scanned. Inthe apparatus, the light beam of the radiation point for radiating thelight beam firstly incident on a deflecting facet of the deflecting unitin the main scanning direction is radiated prior to the light beam fromthe other radiation point.

[0045] Further, that multi-beam optical scanning apparatus can furtherinclude a scanning optical system for forming images of the at leastthree light beams deflected by the deflecting unit on the surface to bescanned, and synchronous detecting unit for detecting writing starttimings on the surface to be scanned by receiving the at least threelight beams deflected by the deflecting unit, and in this apparatus,synchronous detection is performed by radiating the light beam directedto the synchronous detecting unit from the radiation point of the lightsource unit in the upstream-side external angular range prior to theeffective scanning range on the surface to be scanned.

[0046] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having a plurality of radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting a plurality of light beams radiated fromthe plurality of radiation points toward a surface to be scanned. In themulti-beam optical scanning apparatus, where a first radiation point isa radiation point for radiating the light beam, out of the plurality oflight beams emitted from the plurality of radiation points, whichreaches the farthest location from a center of a deflecting facet of thedeflecting unit in the main scanning direction, a second radiation pointis a radiation point for radiating another light beam, and anupstream-side external angular range is a range which lies in an angularrange over which the light beam can be deflected by the deflecting unit,and which exists on an upstream side in a rotational direction of thedeflecting unit relative to an effective scanning angular range at thetime when the light beam is deflected toward an effective scanning rangeon the surface to be scanned, a width of the deflecting facet in a mainscanning section is set to such a magnitude that the light beam reachingthe location most spaced from the center of the deflecting facet at anend portion of the deflecting facet is eclipsed in the event that thelight beam from the first radiation point for radiating the light beamreaching the location most spaced from the center of the deflectingfacet is radiated prior to the light beam from the second radiationpoint in the upstream-side external angular range, and control isperformed such that the light beam from the second radiation point canbe radiated prior to the light beam from the first radiation point inthe upstream-side external angular range.

[0047] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit having a plurality of radiation points disposed withbeing spaced from each other in a main scanning direction, and adeflecting unit for deflecting a plurality of light beams radiated fromthe plurality of radiation points toward a surface to be scanned. In themulti-beam optical scanning apparatus, a width of the deflecting facetin a main scanning section is set to such a magnitude that the lightbeam last incident on an end portion of the deflecting facet is eclipsedin the event that the light beam from the radiation point for radiatingthe light beam last incident on the deflecting facet of the deflectingunit is radiated prior to the light beam from the other radiation point,and the light beam of the radiation point for radiating the light beamfirstly incident on the deflecting facet of the deflecting unit in themain scanning direction is radiated prior to the light beam from theother radiation point.

[0048] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit including at least three radiation points disposedwith being spaced from each other in a main scanning direction, and adeflecting unit for deflecting at least three light beams radiated fromthe at least three radiation points toward a surface to be scanned. Inthe multi-beam optical scanning apparatus, where a first radiation pointis a radiation point for radiating the light beam, out of the at leastthree light beams emitted from the at least three radiation points,which reaches the farthest location from a center of a deflecting facetof the deflecting unit in the main scanning direction, a secondradiation point is a radiation point for radiating another light beam,and an upstream-side external angular range is a range which lies in anangular range over which the light beam can be deflected by thedeflecting unit, and which exists on an upstream side in a rotationaldirection of the deflecting unit relative to an effective scanningangular range at the time when the light beam is deflected toward aneffective scanning range on the surface to be scanned, a width of thedeflecting facet in a main scanning section is set to such a magnitudethat the light beam reaching the farthest location from a center of thedeflecting facet at an end portion of the deflecting facet is eclipsedin the event that the light beam from the first radiation point forradiating the light beam reaching the location most spaced from thecenter of the deflecting facet is radiated prior to the light beam fromthe second radiation point in the upstream-side external angular range,and control is performed such that the light beam from the secondradiation point can be radiated prior to the light beam from the firstradiation point in the upstream-side external angular range.

[0049] According to still another aspect of the present invention, thereis provided a multi-beam optical scanning apparatus which includes alight source unit including at least three radiation points disposedwith being spaced from each other in a main scanning direction, and adeflecting unit for deflecting at least three light beams radiated fromthe at least three radiation points toward a surface to be scanned. Inthe multi-beam optical scanning apparatus, a width of a deflecting facetof the deflecting unit in a main scanning section is set to such amagnitude that the light beam last reaching an end portion of thedeflecting facet is eclipsed in the event that the light beam from theradiation point for radiating the light beam last incident on thedeflecting facet of the deflecting unit is radiated prior to the lightbeam from the other radiation point, and the light beam from theradiation point for radiating the light beam firstly incident on thedeflecting facet of the deflecting unit in the main scanning directionis radiated prior to the light beam from the other radiation point.

[0050] In those multi-beam optical scanning apparatuses, the lightsource unit can be comprised of a monolithic semiconductor laser.

[0051] According to still another aspect of the present invention, thereis provided an image forming apparatus which includes the multi-beamoptical scanning apparatus described above, an image bearing memberplaced at the surface to be scanned, a developing unit for developing anelectrostatic latent image, which is formed on the image bearing memberby the light beam scanned by the multi-beam optical scanning apparatus,as a toner image, a transferring unit for transferring the developedtoner image onto a transferring material, and a fixing unit for fixingthe transferred toner image on the transferring material.

[0052] According to still another aspect of the present invention, thereis provided an image forming apparatus which includes the multi-beamoptical scanning apparatus described above, and a printer controller forconverting code data input from an external apparatus into an imagesignal to supply the image signal to the multi-beam optical scanningapparatus.

[0053] According to still another aspect of the present invention, thereis provided a color image forming apparatus which includes a pluralityof multi-beam optical scanning apparatuses each of which includes amulti-beam optical scanning apparatus described above, and a pluralityof image bearing members each of which is placed at the surface to bescanned of the each multi-beam optical scanning apparatus, and whichform images of different colors, respectively.

[0054] According to still another aspect of the present invention, thereis provided a color image forming apparatus which includes a multi-beamoptical scanning apparatus described above, and a printer controller forconverting code data input from an external apparatus into an imagesignal to supply the image signal to the multi-beam optical scanningapparatus.

[0055] These and further aspects and features of the invention willbecome apparent from the following detailed description of preferredembodiments thereof in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]FIG. 1 is a cross-sectional view in a main scanning sectionillustrating a multi-beam optical scanning apparatus of a firstembodiment according to the present invention;

[0057]FIG. 2 is a view schematically illustrating a main portion of themulti-beam optical scanning apparatus of the first embodiment;

[0058]FIG. 3 is a view showing operation of a synchronous detecting unitin the first embodiment according to the present invention;

[0059]FIG. 4 is a view schematically showing a main portion of a lightsource unit in the first embodiment according to the present invention;

[0060]FIG. 5 is a view schematically illustrating the main portion ofthe multi-beam optical scanning apparatus of the first embodiment;

[0061]FIG. 6A is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the firstembodiment;

[0062]FIG. 6B is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the firstembodiment;

[0063]FIG. 6C is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the firstembodiment;

[0064]FIG. 6D is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the firstembodiment;

[0065]FIG. 6E is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the firstembodiment;

[0066]FIG. 7 is a cross-sectional view in a main scanning sectionillustrating a multi-beam optical scanning apparatus of a secondembodiment according to the present invention;

[0067]FIG. 8 is a view schematically illustrating a main portion of themulti-beam optical scanning apparatus of the second embodiment;

[0068]FIG. 9 is a view schematically illustrating a main portion of themulti-beam optical scanning apparatus of the second embodiment;

[0069]FIG. 10 is a cross-sectional view in a main scanning sectionillustrating a multi-beam optical scanning apparatus of a thirdembodiment according to the present invention;

[0070]FIG. 11 is a cross-sectional view in a main scanning sectionillustrating a multi-beam optical scanning apparatus of a fourthembodiment according to the present invention;

[0071]FIG. 12A is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the fourthembodiment;

[0072]FIG. 12B is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the fourthembodiment;

[0073]FIG. 13 is a cross-sectional view in a main scanning sectionillustrating a multi-beam optical scanning apparatus of a fifthembodiment according to the present invention;

[0074]FIG. 14 is a view schematically illustrating a main portion of themulti-beam optical scanning apparatus of the fifth embodiment;

[0075]FIG. 15A is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the fifthembodiment;

[0076]FIG. 15B is a view illustrating a manner in which light beams aredeflected in the multi-beam optical scanning apparatus of the fifthembodiment;

[0077]FIG. 16 is a cross-sectional view in a main scanning sectionillustrating a multi-beam optical scanning apparatus of a sixthembodiment according to the present invention;

[0078]FIG. 17A is a view schematically illustrating a main portion ofthe multi-beam optical scanning apparatus of the sixth embodiment;

[0079]FIG. 17B is a view schematically illustrating a main portion ofthe multi-beam optical scanning apparatus of the sixth embodiment;

[0080]FIG. 17C is a view schematically illustrating a main portion ofthe multi-beam optical scanning apparatus of the sixth embodiment;

[0081]FIG. 18 is a cross-sectional view in a main scanning sectionillustrating a multi-beam optical scanning apparatus of a seventhembodiment according to the present invention;

[0082]FIG. 19 is a cross-sectional view in a sub scanning sectionillustrating an embodiment of an image forming apparatus according tothe present invention;

[0083]FIG. 20 is a schematic view illustrating a main portion of anembodiment of a color image forming apparatus according to the presentinvention; and

[0084]FIG. 21 is a perspective view illustrating a conventionalmulti-beam optical scanning apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

[0085]FIG. 1 is a cross-sectional view in a main scanning directionillustrating a main portion of a multi-beam scanning apparatus of afirst embodiment according to the present invention.

[0086] Here, the main scanning direction means a direction perpendicularto a rotational axis of a deflecting unit and an optical axis of ascanning optical system (i.e., a direction along which a light beam isreflectively deflected (deflection-scanned) by the deflecting unit), andthe sub scanning direction-means a direction parallel to the rotationalaxis of the deflecting unit. Further, the main scanning section means aplane parallel to the main scanning direction and including the opticalaxis of the scanning optical system. The sub scanning section means aplane perpendicular to the main scanning section.

[0087] In FIG. 1, reference numeral 1 represents a light source unitcomprised of a monolithic semiconductor laser array including tworadiation points 1 a and 1 b. The two radiation points 1 a and 1 b arespaced from each other in both the main scanning direction and the subscanning direction. In a scanning apparatus of the present invention,the above light source unit can be replaced by a light source unitincluding three or more radiation points spaced from each other in boththe main scanning direction and the sub scanning direction.

[0088] Reference numeral 2 represents a condensing lens system (acollimator lens) comprised of a single lens. The condensing lens system2 converts two light beams BMa and BMb emitted from the light sourceunit 1 into nearly parallel light beams (or nearly diverging lightbeams, or nearly converging light beams).

[0089] Reference numeral 3 represents a lens system (a cylindrical lens)having a predetermined refractive power only in the sub scanningdirection. Reference numeral 4 represents an aperture stop forrestricting a light beam (the amount of light). The aperture stop 4shapes two light beams BMa and BMb emerging from the cylindrical lens 3into desired optimum beam forms.

[0090] Elements of the collimator lens 2, the cylindrical lens 3, theaperture stop 4 and the like constitute a portion of an incidenceoptical unit.

[0091] Reference numeral 5 represents an optical deflector (serving as adeflecting unit) comprised of a polygon mirror with six facets, forexample, which is rotated at a uniform speed in a direction of an arrowA by a driving unit (not shown) such as a motor.

[0092] Reference numeral 7 represents a scanning optical system whichhas imaging performance and f-φ θ characteristic, and consists of asingle scanning lens. The scanning optical system 7 forms spot-shapedimages of the two light beams BMa and BMb deflected by the opticaldeflector 5 on a photosensitive drum surface 8, and two scanning linesare thus formed on the photosensitive drum surface 8. The scanningoptical system 7 establishes a conjugate relationship between a placeclose to a deflecting facet 5 a of the optical deflector 5 and a placeclose to the photosensitive drum surface 8 in the sub scanning sectionsuch that fall or inclination of the reflective deflection facet 5 a canbe compensated for.

[0093] Reference numeral 8 represents the surface of the photosensitivedrum, which is a surface to be scanned. Reference numeral 8 a representsan effective scanning range.

[0094] Reference numeral 9 represents a folding mirror (a BD mirror) forsynchronous detection, which reflects toward a side of a synchronousdetecting unit 10 two light beams (BD beams) for synchronous detectionand for adjustment of timing of a scanning start position on thephotosensitive drum surface 8.

[0095] Reference numeral 10 represents the synchronous detecting unitwhich includes a slit (a BD slit) 10 a for synchronous detection, and anoptical sensor (a BD sensor) 10 b serving as a synchronous detectingdevice. In the synchronous detecting unit 10, a synchronous signal (a BDsignal) obtained by detection of an output signal from the BD sensor 10b is used to adjust the timing of the scanning start position of imagerecording on the photosensitive drum surface 8. The BD slit 10 a isdisposed at a location which is optically equivalent to thephotosensitive drum surface 8 to determine a writing start position ofan image.

[0096] Elements of the BD mirror 9, the BD slit 10 a, the BD sensor 10and the like constitute a portion of a synchronous detecting opticalsystem (a BD optical system).

[0097] In the first embodiment, two light beams BMa and BMb emitted fromthe light source unit 1 and optically modulated according to imageinformation are converted into nearly parallel light beams by thecollimator lens 2, and are incident on the cylindrical lens 3. Eachlight beam incident on the cylindrical lens 3 emerges therefrom withoutany change in the main scanning section, and passes through the aperturestop 4. The light beam is partially intercepted by the aperture stop 4.With respect to the sub scanning section, each light beam is convergedby the cylindrical lens 3, passes through the aperture stop 4 with aportion of the light beam being intercepted, and is imaged on thedeflecting facet 5 a of the optical deflector 5 as an approximatelylinear image (a linear image extending in the main scanning direction).Each of the light beams BMa and BMb reflectively deflected by thedeflecting facet 5 a of the optical deflector 5 is imaged on thephotosensitive drum surface 8 in the form of a spot by the scanning lens7. The photosensitive drum surface 8 is scanned with the imaged spotmoving at a uniform speed in a direction of an arrow B (the mainscanning direction) when the optical deflector 5 is rotated in adirection of an arrow A. Accordingly, two scanning lines aresimultaneously formed on the photosensitive drum surface 8 serving as arecording material such that image recording can be executed thereon.

[0098] Here, in the main scanning section, widths of the two light beamsBMa and BMb incident on the deflecting facet 5 a are smaller than afacet width of the deflecting facet 5 a (an under-field optical system).

[0099] At the time of the above-discussed operation, a portion (a BDlight beam) of each of the two light beams BMa and BMb reflectivelydeflected by the optical deflector 5 is condensed on the BD slit 10 athrough the BD mirror 9, and then guided to the BD sensor 10 b, so thatthe timing of the scanning start position on the photosensitive drumsurface 8 can be adjusted prior to the optical scanning of thephotosensitive drum surface 8. For each BD light beam, the timing of thescanning start position in the main scanning direction for imagerecording on the photosensitive drum surface 8 is adjusted, using thesynchronous signal (a BD signal) obtained by detecting the output signalfrom the BD sensor 10 b.

[0100]FIG. 2 schematically illustrates a main portion of the multi-beamscanning apparatus of the first embodiment. In FIG. 2, like referencenumerals designate the same elements as those illustrated n FIG. 1.

[0101] In FIG. 2, there is an angular limitation to a range over whichthe deflecting facet 5 a of the optical deflector 5 can deflect thelight beam emitted from the light source unit 1. In FIG. 2, “adeflectable angular range” 8 c represents that limited angular range,and this range is an angular range of 100-% scanning efficiency. A rangeof a deflectable angle θc (rad) is given by

−4π/C≦θc≦4π/C  relation 1

[0102] where C is the number of facets of the optical deflector.

[0103] In the first embodiment, the number C of the facets of theoptical deflector 5 is six (6), and the range of the deflectable angleθc (rad) can be written, from relation 1, as follows

−2π/3≦θc≦2π/3  relation 2

[0104] In the case of the scanning optical system, the deflection angle(an angle of view) on-to-one corresponds to the scan position, andaccordingly when the range is designated, it is possible to use not onlythe deflection angle but also the scan position.

[0105] Normally, “an effective scanning angular range” 8 b is present ina central portion of the above-discussed “deflectable angular range” 8c, and its scanning positional counterpart is “an effective scanningrange” 8 a wherein a latent image is formed on the photosensitive drumsurface 8. Further, on an upstream side in the rotational direction ofthe optical deflector 5 relative to the “effective scanning angularrange” 8 b, there exists “an upstream-side external angular range” 8 dwhich is a portion of the “deflectable angular range” 8 c. A light beam(a BD light beam) BMbd guided toward the synchronous detecting unit 10is present in that external angular range. “A downstream-side externalangular range” 8 e exists on a downstream side, and there can be a casewhere a light beam (a BD light beam) guided toward the synchronousdetecting unit 10 is also present in this downstream-side externalangular range.

[0106] Also in conventional single-beam scanning apparatuses, in theevent that an angular range of a light beam deflected by the opticaldeflector 5 extends to the “upstream-side external angular range” or“downstream-side external angular range” outside the “effective scanningangular range”, such as a case where the light beam is deflected andguided toward the synchronous detecting unit 10, a light beam emittedfrom the light source unit 1 is reflectively deflected using a place inthe vicinity of the end of the deflecting facet 5 a. Therefore, there isnot so much room between the edge of the light beam and the end of thedeflecting facet 5 a.

[0107] The relationship between the width of an incident light beam usedin this embodiment and the optical deflector 5 in the event that theincident light beam is eclipsed by the optical deflector 5 is given by$\begin{matrix}{{{Wm} > {{Wnir} + {Wfar} - {W\max} - {\left( {n - 1} \right) \times \Delta \quad {Tm}} - {2 \times {Pc}}}}{where}{{W\max} = {2 \times {Rp} \times {\sin\left( {\frac{\pi}{Mp} - {\theta y\max}} \right)} \times {\cos\left( \frac{Ai}{2} \right)}}}\text{}{{Wnir} = {2 \times {Rp} \times {\sin \left( {\frac{\pi}{Mp} - {\theta y\max} - \frac{\varphi}{2}} \right)} \times {\cos \left( \frac{{Ai} - \varphi}{2} \right)}}}{{Wfar} = {2 \times {Rp} \times {\sin \left( {\frac{\pi}{Mp} - {\theta y\max} - \frac{\xi}{2}} \right)} \times {\cos \left( \frac{{Ai} + \xi}{2} \right)}}}} & (1)\end{matrix}$

[0108] and Wm is the width (mm) of the incident light beam in the mainscanning direction, n is the number of beams, ΔTm is the deviation (mm)between adjacent light beams (see relation 5 discussed below), Pc is thechamfer amount (mm) on one side, Rp is the radius (mm) of acircumscribed circle of the optical deflector 5 (the polygon mirror), Mpis the number of facets of the optical deflector 5 (the polygon mirror),θymax is the absolute value (rad) of a rotational angle of the opticaldeflector 5 (the polygon mirror) at the maximum image height, Ai is theincident angle (rad) of the incident light beam (the angle between theoptical axis of the incident light beam and the optical axis of thescanning optical system), φ is the angular difference (rad) between theabsolute value of the rotational angle of the optical deflector 5 at thetime when the light beam is deflected to the external angular range onthe side of presence of the incident light beam, and the rotationalangle θymax of the optical deflector 5 at the maximum image height, andξ is the angular difference (rad) between the absolute value of therotational angle of the optical deflector 5 at the time when the lightbeam is deflected to the external angular range on the side of absenceof the incident light beam, and the rotational angle θymax of theoptical deflector 5 at the maximum image height.

[0109] In the first embodiment, n=4, α=0.0038 (rad), Lap=33.00 (mm),ΔTm=0.1238 (mm), Pc=0.21 (mm), Ra=20 (mm), Mp=6, θymax=0.3927 (rad),Ai=1.0472 (rad), φ=0.0349 (rad), and ξ=0.0349 (rad). This will bereferred to as condition A. Ray eclipse of the incident light beamoccurs from relation (1) when the width Wm of the incident light beam inthe main scanning direction is Wm>2.78 (mm).

[0110] The ray eclipse can occur even if the above condition is notsatisfied. The reason therefor is that there is a need to preferablycorrect aberration at a position of a high image height by disposing theoptical deflector 5 in a position close to a 100-% crossing position,rather than by disposing the optical deflector 5 in such a manner thatthe light beam can be most widely reflectively deflected.

[0111] Further, in the event that the optical deflector 5 is disposed inthe 100-% crossing position, it is likely to cause the problem that thewidth of the light beam capable of being reflectively deflected isnarrow. Therefore, the optical deflector 5 is often disposed beingshifted from the 100-% crossing position.

[0112] In the following, there is shown the relationship between thearrangement of the deflector (the polygon mirror) and the width Wm ofthe incident light beam in the main scanning direction at the time whenthe ray eclipse occurs, in the event that the optical deflector 5 isdisposed being shifted from the 100-% crossing position.

Wm>(Dcloss+Wnir−Wmax−Pc+ΔYbm)×2−(n−1)×ΔTm  (2)

[0113] or $\begin{matrix}{{{Wm} > {{\left( {{Wfar} - {Dcloss} - {Pc} - {\Delta \quad {Ybm}}} \right) \times 2} - {\left( {n - 1} \right) \times \Delta \quad {Tm}}}}{where}{{Dcloss} = {{Rp} \times {\sin\left( {\frac{\pi}{Mp} - {\theta \quad {y\max}}} \right)} \times \frac{\cos\left( {{\theta \quad {y\max}} + \frac{Ai}{2}} \right)}{\cos\left( {\theta \quad {y\max}} \right)}}}} & (3)\end{matrix}$

[0114] and ΔYbm is the amount of shift of the principal ray of theincident light beam from a 100-% crossing point in a directionperpendicular to the incident light beam (a direction in which theprincipal ray of the incident light beam approaches the optical axis ofthe scanning optical system is a plus direction). The 100-% crossingpoint is defined by a crossing point between deflecting facets underconditions under each of which the incident light beam is deflected tothe maximum image height on each of opposite sides.

[0115] In the event that the amount ΔYbm of shift of the incident lightbeam is ΔYbm=0.50 (mm) in addition to the above-discussed condition A,the incident light beam is eclipsed by the optical deflector 5 whenWm>2.77 (mm), as can be understood from relation (2). The incident lightbeam is eclipsed by the optical deflector 5 when Wm>2.78 (mm), as can beunderstood from relation (3).

[0116] In the first embodiment, as described later, where a firstradiation point is defined by a radiation point for radiating a lightbeam, out of two light beams BMa and BMb emitted from the light sourceunit 1, that reaches the farthest location from a center of thedeflecting facet of the optical deflector 5 in the main scanningdirection, and a second radiation point is defined by a radiation pointfor radiating another light beam, the second radiation point is excitedin the first place such that a light beam therefrom can be in theupstream-side external angular range 8 d in the deflectable angularrange 8 c of the optical deflector 5, which lies on the upstream side inthe rotational direction of the optical deflector 5 relative to theeffective scanning angular range 8 b at the time when the light beam isdeflected and guided toward the effective scanning range 8 a on thephotosensitive drum surface 8.

[0117]FIG. 3 shows a manner of optical detection in the synchronousdetecting unit 10. In the synchronous detecting unit 10, the amount oflight passing through the BD slit 10 a and detected by the BD sensor 10b increases with time, as indicated by a solid line a in FIG. 3, and amoment (time) at which the light amount reaches a threshold is detected.Radiation is started at a time after a predetermined time of period fromthe detection moment such that the scanning start positions in the mainscanning direction on the scanned surface 8 for the two light beams canbe aligned with each other.

[0118]FIG. 4 illustrates the arrangement of the two radiation points 1 aand 1 b in the light source unit 1. In the multi-beam scanning apparatusof the first embodiment, the light source unit 1 is rotated by an angleδ about the optical axis and adjusted such that the distance betweenscanning lines on the scanned surface 8 can be a predetermined distancecorresponding to the density of pixels. The two radiation points 1 a and1 b are thus spaced from each other by a distance Ds in the mainscanning direction, and also spaced by a distance Dm in the sub scanningdirection.

[0119]FIG. 5 schematically illustrates a main portion in the mainscanning direction of the multi-beam scanning apparatus of the firstembodiment. In FIG. 5, like reference numerals designate the sameelements as those illustrated in FIG. 1. FIG. 5 schematicallyillustrates a manner in which light beams emitted from the light sourceunit 1 are reflectively deflected toward the synchronous detecting unit10 by the optical deflector 5.

[0120] In the main scanning direction of the multi-beam scanningapparatus of the first embodiment, the two radiation points 1 a and 1 bare spaced from each other in the main scanning direction, and the twolight beams BMa and BMb are incident on the deflecting facet 5 a atdifferent incident angles, so that principal rays of the two light beamsBMa and BMb radiated from the respective radiation points la and 1 breach positions on the deflecting facet 5 a spaced from each other inthe main scanning direction. Accordingly, when the two light beams BMaand BMb radiated from the light source unit 1 are deflected by thedeflecting facet 5 a of the optical deflector 5 toward the synchronousdetecting unit 10, for example, there exist the light beam BMa(indicated by a solid line), out of the two light beams, that reachesthe location near the center of the deflecting facet 5 a, and the lightbeam BMb (indicated by a dashed line) that reaches the location far fromthe center of the deflecting facet 5 a. With the light beam BMb reachingthe location far from the center of the deflecting facet 5 a, there is afear that its light cannot be entirely placed on the deflecting facet 5a and eclipsed by the deflecting facet 5 a.

[0121] In the event that the light beam BMb eclipsed by the deflectingfacet 5 a is incident on the synchronous detecting unit 10, the amountof light detected by the BD sensor 10 b decreases. Then, it takes longertime the light amount to reach the threshold as illustrated by a dashedline b in FIG. 3. Accordingly, the problem occurs that timing is delayedand the scanning start position deviates. Particularly, in the case ofthe multi-beam scanning apparatus, the amount of light eclipsed by thedeflecting facet 5 a, is likely to vary between individual light beams,and the scanning start position of the scanning line for each light beamfluctuates, leading to degradation of the image quality. Therefore, theeclipse of the light beam from the light source unit 1 by the deflectingfacet 5 a is a critical problem.

[0122] Such a problem is serious in the multi-beam scanning apparatuswith high resolution. For example, the problem is very serious in ascanning apparatus with over 600 dpi. Especially, the problem is moreserious in a multi-beam scanning apparatus usable in a color imageforming apparatus with over 1200 dpi.

[0123] Further, the present invention exhibits preeminent advantageswhen applied to a tandem-type color image forming apparatus (describedlater with reference to FIG. 20) provided with a plurality of surfacesto be scanned. The reason therefor is that color shift (registrationshift) due to the shift in scanning start positions of plural lightbeams incident on the same scanned surface is a serious problem in thetandem-type color image forming apparatus in which plural light beamsare incident on different scanned surfaces (for example, photosensitivedrums) to form images of different colors on the different scannedsurfaces, respectively, and superimposition of the images of differentcolors are then performed on a sheet of paper.

[0124] However, if the size of the deflecting facet 5 a is increased,not only the size of the optical deflector 5 but also torque of adriving unit for rotating the optical deflector 5 are enlarged.Disadvantages of cost are hence incurred. The first embodiment of thepresent invention therefore solves the above problem by effectivelyusing the deflecting facet 5 a.

[0125] In other words, the apparatus excites the radiation point forradiating the light beam, out of the two light beams emitted from thelight source unit 1, which is firstly incident on the deflecting facet 5a of the optical deflector 5 in the main scanning direction, prior toanother radiation point.

[0126] Specifically, the apparatus firstly excites the radiation point(the second radiation point) 1 a for radiating the light beam BMa thatreaches the location nearest the center of the deflecting facet 5 a, outof the two light beams BMa and BMb reaching the deflecting facet 5 awhen the light beam is guided to the synchronous detecting unit 10, toperform the synchronous detection (BD detection), and the apparatus thenexcites the radiation point (the first radiation point) 1 b forradiating the light beam BMb that reaches the location away from thecenter of the deflecting facet 5 a, after the optical deflector 5rotates in the direction of arrow A to move the deflecting facet 5 a toa state in which the light beam is no more eclipsed thereby, to performthe synchronous detection.

[0127] Here, though locations for achieving the synchronous detectiondiffer between the two light beams BMa and BMb, this difference can bereadily coped with by providing slits 10 a of the synchronous detectingunit 10 at two places. Alternatively, synchronous detecting units 10 canbe provided for the two light beams BMa and BMb, respectively.

[0128]FIGS. 6A to 6E schematically illustrate manners in which the twolight beams BMa and BMb reach the deflecting facets 5 a, respectively.In FIGS. 6A to 6E, like reference numerals designate the same elementsas those illustrated in FIG. 1. In the first embodiment, the two lightbeams BMa and BMb emitted from the light source unit 1 reach differentlocations on the deflecting facet 5 a, respectively.

[0129]FIG. 6A schematically illustrates the manner in which the lightbeam BMa from the radiation point (the second radiation point) 1 a isdeflected and guided to the synchronous unit (not shown). Under thiscondition, the entire light of the light beam BMa is reflectivelydeflected by the deflecting facet 5 a, while the light beam BMb from theradiation point (the first radiation point) 1 b strides end portions ofthe adjacent deflecting facets 5 a and a portion of the light beam BMbis eclipsed.

[0130]FIG. 6B schematically illustrates the manner in which the lightbeam BMb is deflected and guided to the synchronous unit (not shown).Under this condition, the entire light of the light beam BMb is alsoreflectively deflected by the deflecting facet 5 a.

[0131]FIG. 6C schematically illustrates the manner in which the twolight beams are deflected and guided to an area within the effectivescanning range on the scanned surface. Under this condition, both thelight beams BMa and BMb reach a place near a central portion of thedeflecting facet 5 a.

[0132]FIG. 6D schematically illustrates the manner in which the twolight beams are deflected and guided to a location beyond the effectivescanning range on the scanned surface. Under this condition, the lightbeam BMa (not the light beam BMb) reaches a location away from thecenter of the deflecting facet 5 a, while the light beam BMb (not thelight beam BMa) reaches a location near the center of the deflectingfacet 5 a.

[0133]FIG. 6E schematically illustrates a condition immediately afterthe condition illustrated in FIG. 6D. Under this condition, the lightbeam BMa strides end portions of the adjacent deflecting facets 5 a anda portion of the light beam BMa is eclipsed, while the entire light ofthe light beam BMb is reflectively deflected by the deflecting facet 5a.

[0134] Conditions of FIGS. 6A to 6E occur in a time sequence of FIG.6A→FIG. 6B→FIG. 6C→FIG. 6D→FIG. 6E. The light beams BMa and BMb move onthe deflecting facet 5 a in such an order. Although the two light beamsBMa and BMb do not actually move, these light beams move relative to thedeflecting facet 5 a since the optical deflector 5 rotates.

[0135] Only the light beam BMa can be reflectively deflected in itsentirety under the condition of FIG. 6A, and after a little time elapsesand the condition of FIG. 6B is established, the light beam BMb can alsobe reflectively deflected in its entirety.

[0136] In the first embodiment, the synchronous detection is executed atthe moment when the entire light beam can be reflectively deflected bythe deflecting facet 5 a, and radiation order is set for the purposes ofsynchronous detection in such a manner that the radiation point (thesecond radiation point) 1 a for radiating the light beam BMa is excitedin the first place, and the radiation point (the first radiation point)1 b for radiating the light beam BMb is then excited.

[0137] Further, in the first embodiment, the two light beams BMa and BMbintersect each other at the aperture stop 4, and only this crossingpoint is present between the light source unit 1 and the deflectingfacet 5 a. Accordingly, the number M of crossing points is M=2×n+1=1(n=0). In this case, a light beam reaching the place near the center ofthe deflecting facet 5 a prior to the scanning start is the light beamBMa from the radiation point 1 a located downstream in the rotationaldirection A of the optical deflector 5, and a light beam reaching theplace away from the center of the deflecting facet 5 a is the light beamBMb from the radiation point 1 b located upstream in the rotationaldirection A of the optical deflector 5.

[0138] Therefore, in the first embodiment, where the first radiationpoint is defined by the radiation point for radiating the light beamlocated on the most upstream side in the rotational direction of theoptical deflector 5, and the second radiation point is defined by theradiation point for radiating the light beam located on the downstreamside, as described above, the second radiation point 1 a for radiatingthe light beam BMa is excited in the first place for the synchronousdetection, and the first radiation point 1 b for radiating the lightbeam BMb is then excited for the synchronous detection. Thereby, astable synchronous detection can be performed without increasing thesize of the optical deflector 5, and hence a compact multi-beam scanningapparatus can be achieved with a space of its construction beingreduced. Further, an optical deflector similar to that usable in asingle-beam apparatus can be employed, and increase in the cost of thedriving unit can be oppressed. Thus, advantages in the cost can beobtained.

[0139] In other words, utilizing the advantageous effects of the firstembodiment, there can be provided a multi-beam scanning apparatus inwhich the size is reduced, a preferable image can be stably obtained,and the structure is simplified.

[0140] As described above, in the first embodiment, the radiation point(the second radiation point) 1 a located on the most downstream side inthe rotational direction of the optical deflector 5 is excited in thefirst place for the synchronous detection, and the radiation point (thefirst radiation point) 1 b is then excited for the synchronousdetection, thereby effectively using the deflecting facet and reducingthe size of the optical deflector. Further, torque necessary for thedriving unit for driving the optical deflector can be decreased.Moreover, ghost light can be prevented from occurring due to thechamfered portion of the optical deflector and the like, and hence apreferable image can be always obtained.

[0141] Although the BD light beam is present in the external angularrange on the upstream side in the above-discussed embodiment, it ispossible to situate the BD light beam in the external angular range onthe downstream side.

Second Embodiment

[0142]FIG. 7 is a cross-sectional view in the main scanning directionillustrating a multi-beam scanning apparatus of a second embodimentaccording to the present invention. In FIG. 7, like reference numeralsdesignate the same elements as those illustrated in FIG. 1.

[0143] The second embodiment is different from the above-discussed firstembodiment in that four radiation points are provided in a light sourceunit 1. Other construction and optical functions are approximately thesame as those of the first embodiment, and similar technical advantagesare achieved in the second embodiment.

[0144] In FIG. 7, the light source unit 1 is comprised of a monolithicsemiconductor laser array having four radiation points 1 a, 1 b, 1 c andid, for example. The four points 1 a, 1 b, 1 c and 1 d are spaced fromeach other in both the main scanning direction and the sub scanningdirection.

[0145]FIG. 8 schematically illustrates a manner in which four lightbeams BMa, BMb, BMc and BMd from the light source unit 1 reach thedeflecting facet 5 a. FIG. 9 is a view illustrating principal rays(central rays of the light beams) of the four light beams BMa, BMb, BMcand BMd illustrated in FIG. 8 only. In FIGS. 8 and 9, like referencenumerals designate the same elements as those illustrated in FIG. 7.

[0146] In the second embodiment, the four light beams BMa (a solidline), BMb (a shorter dashed line), BMc (an alternate long and two shortdashes line) and BMd (a longer dashed line) emitted from the lightsource unit 1 intersect each other at the aperture stop 4, and enter thedeflecting facet 5 a of the optical deflector 5. Also in the secondembodiment, the light beams intersect each other only once in a rangebetween the light source unit 1 and the deflecting facet 5 a of theoptical deflector 5.

[0147] The diameter of an inscribed circle of the optical deflector 5 is40 mm in the second embodiment.

[0148] In the second embodiment, similar to the first embodiment, thefour light beams BMa, BMb, BMc and BMd are incident on the deflectingfacet 5 a at different incident angles, so that principal rays of thefour light beams BMa, BMb, BMc and BMd reach different locations on thedeflecting facet 5 a of the optical deflector 5 serving as therotational polygon mirror. Accordingly, the four light beams BMa, BMb,BMc and BMd reach the deflecting facet 5 a over a wide range in the mainscanning section.

[0149] Here, a spread Tm (mm) of the four light beams BMa, BMb, BMc andBMd reaching the deflecting facet 5 a will be discussed. In the secondembodiment, the spread Tm is larger than that of the two light beams BMaand BMb in the first embodiment, and hence the problem of eclipse of thelight beam by the deflecting facet 5 a is more outstanding in the secondembodiment. That is, the larger the number of the light beams, thelonger the magnitude of the spread Tm. Accordingly, the problem of theeclipse at the end portion of the deflecting facet 5 a must be treatedand solved.

[0150] The spread Tm (mm) of the four light beams can be represented asfollows, using the width Wm (mm) of the light beam in the main scanningdirection, the deviation ΔTm (mm) between the adjacent light beams, thenumber n of the light beams, and the incident angle y (rad) of the lightbeam incident on the deflecting facet 5 a $\begin{matrix}{{Tm} = \frac{{Wm} + {\left( {n - 1} \right) \times \Delta \quad {Tm}}}{\cos \gamma}} & {{relation}\quad 3}\end{matrix}$

[0151] The four light beams BMa, BMb, BMc and BMd emitted from therespective radiation points 1 a, 1 b, 1 c and 1 d are not parallel, andform an angle α therebetween. In the main scanning direction subsequentto the light condensing lens system 2, the angle α (rad) between theadjacent light beams can be written as follows, using the distance Dm(mm) in the main scanning direction between the adjacent radiationpoints, and the focal length fcol (mm) of the light condensing lenssystem 2 $\begin{matrix}{\alpha = \frac{Dm}{fcol}} & {{relation}\quad 4}\end{matrix}$

[0152] The light beams emerge from the light condensing lens system 2,pass through the cylindrical lens 3, form the angle a therebetween andintersect each other at the time when reaching the aperture stop 4, andenter the optical deflector 5 with the angle a being maintained.

[0153] In connection with the deviation ΔTm (mm) between the adjacentlight beams, the distance Lap (mm) between a location ofbeam-intersection locations nearest to the deflecting facet 5 a and thedeflecting facet 5 a is important. In the second embodiment, thelocation of the beam-intersection locations nearest to the deflectingfacet 5 a is the location of the aperture stop 4. The aperture stop 4cannot be disposed in the vicinity of the deflecting facet 5 a due tophysical reasons of the structure.

[0154] Here, the deviation ΔTm (mm) between the adjacent light beams canbe represented as follows, using the distance Lap (mm) between theaperture stop 4 and the deflecting facet 5 a, and the angle a (rad)between the adjacent light beams

ΔTm=Lap×α  relation 5

[0155] The spread Tm (mm) of the four light beams can be given fromrelations 6 and 5 $\begin{matrix}{{Tm} = \frac{{Wm} + {\left( {n - 1} \right) \times {Lap} \times \alpha}}{\cos \gamma}} & {{relation}\quad 6}\end{matrix}$

[0156] Alternatively, the deviation ΔTm (mm) between the adjacent lightbeams can be given from relations 4 and 5 $\begin{matrix}{{\Delta \quad {Tm}} = \frac{{Lap} \times {Dm}}{fcol}} & {{relation}\quad 7}\end{matrix}$

[0157] Accordingly, the spread Tm (mm) of the four light beams can begiven from relations 3 and 7 $\begin{matrix}{{Tm} = \frac{{Wm} + {\left( {n - 1} \right) \times {Lap} \times {Dm}}}{{\cos \gamma} \times {fcol}}} & {{relation}\quad 8}\end{matrix}$

[0158] To paraphrase the above, in the multi-beam scanning apparatushaving four radiation points spaced apart in the main scanningdirection, the spread Tm (mm) of the four light beams on the deflectingfacet 5 a increases by amounts of deviation between the adjacent lightbeams, and therefore a larger deflecting facet 5 a is needed thereinthan in the single-beam scanning apparatus. This means that the size ofthe optical deflector 5 increases, the size of the multi-beam scanningapparatus increases, and torque of the motor for the polygon mirrorshould be enlarged in order to speedily rotate a large heavy opticaldeflector (a polygon mirror), leading to the problem of increase in thecost. Further, the size of the deflecting facet 5 a needs to be enlargedas the number n of the light beams increases.

[0159] Therefore, in the second embodiment, since the light beam BMdemitted from the radiation point (the first radiation point) id arrangedon the most upstream side in the rotational direction A of the opticaldeflector 5 reaches the location most remote from the center of thedeflecting facet 5 a, another light beam is radiated in the first place.It is preferable to radiate the light beam reaching the place nearestthe center of the deflecting facet 5 a, and accordingly the radiationpoint (the fourth radiation point) 1 a disposed on the most downstreamside in the rotational direction A of the optical deflector 5 is excitedin the first place for the synchronous detection. Next, the light beamfrom the radiation point (the third radiation point) 1 b is radiated,the light beam from the radiation point (the second radiation point) 1 cis then radiated, and the light beam from the radiation point (the firstradiation point) 1 d is last radiated. Thus, light radiation issequentially executed from the radiation point 1 a disposed on the mostdownstream side in the rotational direction A of the optical deflector 5to achieve the synchronous detection.

[0160] In other words, the deflecting facet 5 a can be effectively usedwhen the light beam other than the light beam reaching the location mostremote from the center of the deflecting facet 5 a of the opticaldeflector 5 is radiated in the first place. It is preferable to excitethe radiation point (the fourth radiation point) 1 a for radiating thelight beam reaching the location nearest the center of the deflectingfacet 5 a in the first place. It is more preferable to sequentiallyexcite the radiation points beginning with the radiation point (thefourth radiation point) 1 a for radiating the light beam reaching thelocation nearest the center of the deflecting facet 5 a.

[0161] The deflecting facet 5 a of the optical deflector 5 serving asthe rotational polygon mirror can be thus effectively employed, and asmall-sized optical deflector 5 can be used. Increase in the cost of thedriving unit, such as the motor, can also be prevented. Namely,downsizing of the optical deflector 5 and reduction in the cost can beachieved.

[0162] Though the number of light beams to be scanned at a time is fourin the above-discussed apparatus, the present invention can also beapplied to apparatuses using five or more light beams. The same or moreadvantageous effects can also be obtained in such multi-beam scanningapparatuses using multiple light beams such as eight light beams,sixteen light beams, or thirty-two light beams.

Third Embodiment

[0163]FIG. 10 is a cross-sectional view in the main scanning directionillustrating a multi-beam scanning apparatus of a third embodimentaccording to the present invention. In FIG. 10, like reference numeralsdesignate the same elements as those illustrated in FIG. 1.

[0164] The third embodiment is different from the above-discussed firstembodiment in that a light beam (a BD light beam) deflected and guidedto a synchronous detecting unit 10 by the optical deflector 5 serving asthe rotational polygon mirror does not pass through the scanning lens 7,and is instead transmitted through another lens (a synchronous detectingoptical system) 72 and is guided to the synchronous detecting unit 10.Other construction and optical functions are approximately the same asthose of the first embodiment, and similar technical advantages areachieved in the third embodiment.

[0165] In FIG. 10, the synchronous detecting optical system 72 iscomprised of a single lens (a BD lens) having anamorphic refractivepower. The synchronous detecting optical system 72 forms images of thetwo BD light beams on the surface of a BD slit 10 a provided near a BDsensor 10 b.

[0166] In the third embodiment, the BD lens 72 other than the scanninglens 7 is provided in the optical path from the optical deflector 5 tothe synchronous detecting unit 10, and the focal length of the BD lens72 is set shorter than the scanning lens 7, so that the length of theoptical path from the optical deflector 5 to the synchronous detectingunit 10 can be made short.

[0167] Further, since the BD slit 10 a of the synchronous detecting unit10 is disposed on the optical axis of the BD lens 72, the synchronousdetection can be accomplished without any adverse influence due todifference in wavelengths of the light beams which is the problemspecific to the multi-beam apparatus. Accordingly, the scanning startposition can be stabilized. Further, in the event that the radiationpoints 1 a and 1 b and the BD sensor 10 b are arranged on a commonsubstrate, the number of components and arrangement space can bereduced. Furthermore, when the BD lens 72 and the cylindrical lens 3 areformed with a plastic lens in a united form, the cost can be reduced.

[0168] It is thus greatly advantageous to provide the BD lens 72separate from the scanning lens 7 in the optical path between theoptical deflector 5 and the synchronous detecting unit 10.

[0169] In the third embodiment, however, as compared with the multi-beamscanning apparatus as described in the first embodiment in which thelight beam transmitted through the scanning lens 7 is guided to thesynchronous detecting unit 10, it is necessary to widen an angle θbd (anangle of view) at the time when the two light beams BMa and BMb emittedfrom the light source unit 1 are deflected toward the synchronous unit10 by the optical deflector 5, such that these two light beams can beprevented from being guided to the scanning lens 7. Accordingly, a userange of the deflecting facet 5 a of the optical deflector 5 inevitablyincreases. Therefore, the light beam reaching the deflecting facet 5 ais further likely to get to a place more remote from the center of thedeflecting facet 5 a, and the optical deflector 5 needs to be enlarged.

[0170] Also in the third embodiment, such problem is solved byappropriately determining the radiation order of the radiation pointsfor the synchronous detection, similar to the above-discussed firstembodiment.

[0171] In the third embodiment, the two light beams BMa and BMb emittedfrom the light source unit 1 intersect each other at the aperture stop4, the light beams intersect each other only once in a range between thelight source unit 1 and the deflecting facet 5 a. Accordingly, theradiation point 1 a (the second radiation point) for radiating the lightbeam BMa disposed on the downstream side in the rotational direction Aof the optical deflector 5 is excited in the first place to perform thesynchronous detection, and the radiation point 1 b (the first radiationpoint) for radiating the light beam BMb disposed on the upstream side inthe rotational direction of the optical deflector 5 is then excited toperform the synchronous detection.

[0172] Thus, also in the multi-beam scanning apparatus provided with theoptical path directed to the synchronous detecting unit 10 separatelyfrom the optical path directed to the scanning lens 7, the synchronousdetection can be stably performed without need of increasing the size ofthe optical deflector 5.

Fourth Embodiment

[0173]FIG. 11 is a cross-sectional view in the main scanning directionillustrating a multi-beam scanning apparatus of a fourth embodimentaccording to the present invention. In FIG. 11, like reference numeralsdesignate the same elements as those illustrated in FIG. 1.

[0174] The fourth embodiment is different from the above-discussed firstembodiment in that an optical deflector 5 serving as the rotationalpolygon mirror is rotated in a reverse direction, and a synchronousdetecting unit 10 and an optical system (a BD lens) 72 for synchronousdetection are disposed on a side opposite to the light source unit 1with respect to the optical axis L of the scanning lens 7. Otherconstruction and optical functions are approximately the same as thoseof the first embodiment, and similar technical advantages are achievedin the fourth embodiment.

[0175] In the multi-beam scanning apparatus of the fourth embodiment inFIG. 11, the light source unit 1 includes two radiation points 1 a and 1b spaced from each other in the main scanning direction, and two lightbeams BMa and BMb emitted from the respective radiation points 1 a and 1b are transmitted through the light condensing lens 2 and thecylindrical lens 3, intersect each other at the aperture stop 4, and areincident on the deflecting facet 5 a of the optical deflector 5.

[0176] In the fourth embodiment, the optical deflector 5 is rotated at auniform speed in a direction of an arrow A (a direction opposite to thedirection of the first embodiment) in FIG. 11, and the two light beamsBMa and BMb emitted from the light source unit 1 are scanned in adirection of an arrow B on the photosensitive drum surface 8. A portion(the BD light beam) of the light beam deflected by the optical deflector5 is transmitted through the BD lens 72 and guided to the synchronousdetecting unit 10 such that the scanning start position in the mainscanning direction on the photosensitive drum surface 8 can bedetermined.

[0177] Here, in the event that the light beam emitted from the lightsource unit 1 is deflected by the optical deflector 5, a side ofdeflection on the side of the light source unit 1 with respect to theoptical axis L of the scanning lens 7 is a “plus” side, and a side ofdeflection on a side opposite to the light source unit 1 with respect tothe optical axis L of the scanning lens 7 is a “minus” side.

[0178] When the light beam emitted from the light source unit 1 isdeflected on the minus side θ(−) by the optical deflector 5, the lightbeam from the light source unit 1 reaches a place more away from thecenter of the deflecting facet 5 a than when the light beam is deflectedon the plus side θ(+) by the optical deflector 5. In other words, roomof the deflecting facet 5 a of the optical deflector 5 is smaller on theminus side θ(−) than on the plus side θ(+).

[0179] In the multi-beam scanning apparatus constructed such that thelight beam from the light source unit 1 is deflected on the minus sideby the optical deflector 5 when guided to the synchronous detecting unit10 as in the fourth embodiment, room is short on the deflecting facet 5a, and the light beam is more likely to be eclipsed.

[0180] In the fourth embodiment, therefore, the radiation order of thelight beams during the synchronous detection is appropriately set suchthat the synchronous detection can be stably performed without anyeclipse.

[0181]FIGS. 12A and 12B illustrate manners in which the two light beamsBMa and BMb reach the deflecting facet 5 a, respectively. In FIGS. 12Aand 12B, like reference numerals designate the same elements as thoseillustrated in FIG. 1. In the fourth embodiment, the two light beams BMaand BMb emitted from the light source unit 1 reach different locationson the deflecting facet 5 a.

[0182]FIG. 12A illustrates a manner in which the light beam BMb(indicated by a dashed line) is deflected toward the synchronousdetecting unit (not shown). The light beam BMa (indicated by a solidline) strides end portions of the adjacent deflecting facets 5 a and aportion thereof is eclipsed, while the entire light beam BMb (indicatedby the dashed line) is reflectively deflected by the deflecting facet 5a.

[0183]FIG. 12B illustrates a manner in which the light beam BMa(indicated by the solid line) is deflected toward the synchronousdetecting unit a little after the state of FIG. 12A. The entire lightbeam BMb (indicated by the dashed line) is also reflectively deflectedby the deflecting facet 5 a.

[0184] In the fourth embodiment, the arrangement of the radiation pointsis the same as the third embodiment with the exception that theradiation point (the first radiation point) 1 a and the radiation point(the second radiation point) 1 b are disposed on the upstream side andthe downstream side in the rotational direction, respectively.Accordingly, the radiation point 1 b for radiating the light beam BMb isradiated in the first place to perform the synchronous detection, andthe radiation point 1 a for radiating the light beam BMa is thenradiated to perform the synchronous detection.

[0185] Thus, in the fourth embodiment, the deflecting facet 5 a of theoptical deflector 5 can be effectively used, and increase in the size ofthe optical deflector 5 can be prevented also in the event thatdeflection is conducted beyond the effective scanning range on the minusside. In other words, the optical deflector can be downsized, and acompact multi-beam scanning apparatus can be achieved at relatively lowcosts.

Fifth Embodiment

[0186]FIG. 13 is a cross-sectional view in the main scanning directionillustrating a multi-beam scanning apparatus of a fifth embodimentaccording to the present invention. In FIG. 13, like reference numeralsdesignate the same elements as those illustrated in FIG. 1.

[0187] The fifth embodiment is different from the above-discussed firstembodiment in that chamfered portions 5 b are formed at ends (ridges) ofthe deflecting facets 5 a of the optical deflector 5 serving as therotational polygon mirror. Other construction and optical functions areapproximately the same as those of the first embodiment, and similartechnical advantages are achieved in the fifth embodiment.

[0188] In the multi-beam scanning apparatus of the fifth embodiment inFIG. 13, two radiation points 1 a and 1 b of the light source unit 1 forradiating light beams BMa and BMb are started to be excited immediatelybefore each light beam is guided to the synchronous detecting unit 10(to the upstream-side external angular range), and the radiation amountis regulated (APC). After the radiation amount is regulated to a desiredvalue by the APC and stabilized, the light beam is guided to thesynchronous detecting unit 10 to stably perform the synchronousdetection.

[0189] In the fifth embodiment, the light source unit 1 is comprised ofa monolithic semiconductor laser array in which radiation amounts oflight beams emitted from two radiation points 1 a and 1 b are measuredby a measuring device provided in the laser device. The radiationamounts are adjusted to desired values based on measured resultssupplied from the measuring device.

[0190] The chamfered portion 5 b can be provided at the end portion ofthe deflecting facet 5 a of the optical deflector 5 as a chuckingportion of the optical deflector 5 at the time of its fabrication, orfor the purposes of reducing wind whistling sound of the opticaldeflector. In the fifth embodiment, the chamfered portion 5 b is formedalong a circle with a diameter of 39 mm which is a little smaller thanthe diameter 40 mm of the circumscribed circle of the optical deflector5.

[0191] In recent years, the angle of view of the scanning lens 7 tendsto increase as the size of the multi-beam scanning apparatus decreases,and accordingly there is only a small room on the deflecting facet 5 aof the optical deflector 5. Particularly, a portion close to the end ofthe deflecting facet 5 a is often used in the event that the light beamis guided to the synchronous detecting unit 10 provided outside theeffective scanning range on the photosensitive drum surface 8.

[0192] The APC (auto power control) is generally performed immediatelybefore the synchronous detection. There is a case where the light beamis emitted from the light source unit 1 to perform the APC under acondition under which the light beam threatens to be eclipsed by thedeflecting facet 5 a of the optical deflector 5, but no problem occurseven unless the light beam is incident on the optical deflector 5because measurement of the light amount in the APC is executed in thelaser device.

[0193] However, when the chamfered portion 5 b is formed at the endportion of the deflecting facet 5 a of the optical deflector 5, room ofthe deflecting facet 5 a is likely to be still smaller. FIG. 14 is across-sectional view in the main scanning direction illustrating ghostlight BMgh appearing at the chamfered portion 5 b of the opticaldeflector 5.

[0194] As illustrated in FIG. 14, the light beam emitted from the lightsource unit 1 strides over the chamfered portion 5 b, and the ghostlight beam BMgh reflected by the chamfered portion 5 b reaches thephotosensitive drum surface 8. Thus, the problem of black streaks of animage is likely to occur.

[0195] In the fifth embodiment, therefore, the radiation order of thelight beams is likewise appropriately designed during the time of APC toprevent the occurrence of the ghost light BMgh due to the chamferedportion 5 b.

[0196]FIGS. 15A and 15B illustrate manners in which the two light beamsBMa and BMb reach the deflecting facet 5 a, respectively. In FIGS. 15Aand 15B, like reference numerals designate the same elements as thoseillustrated in FIG. 1. In the fifth embodiment, the two light beams BMaand BMb emitted from the light source unit 1 reach different locationson the deflecting facet 5 a.

[0197] As illustrated in FIG. 15A, out of the two light beams BMa andBMb emitted from the light source unit 1, the radiation point (thesecond radiation point) 1 a for radiating the light beam BMa reachingthe location nearest the center of the deflecting facet 5 a of theoptical deflector 5 is excited in the first place to perform the APCunder a condition under which the light beam does not stride over thechamfered portion 5 b of the deflecting facet 5 a. Then, as illustratedin FIG. 15B, the other radiation point (the first radiation point) 1 bfor radiating the light beam BMb is excited to perform the APC at themoment when the optical deflector 5 is rotated so as to establish thecondition under which the light beam does not stride over the chamferedportion 5 b of the deflecting facet 5 a.

[0198] In the fifth embodiment, the two light beams BMa and BMb emittedfrom the two radiation points 1 a and 1 b of the light source unit 1intersect each other only once in a range between the light source unit1 and the optical deflector 5. The radiation point (the second radiationpoint) 1 a for radiating the light beam BMa disposed on the downstreamside in the rotational direction A of the optical deflector 5 is excitedin the first place, and the radiation point (the first radiation point)1 b for radiating the light beam BMb disposed on the upstream side inthe rotational direction is then excited.

[0199] There can thus be provided a multi-beam scanning apparatuscapable of always obtaining a favorable image without any ghost whileusing a compact optical deflector 5. Here, the radiation order of thelight beams for the synchronous detection is opposite to that for theAPC.

[0200] The time sequence of the radiation order is as follows. Theradiation point 1 a for radiating the light beam BMa is initiallyexcited to perform the APC, and is then turned off. The radiation point1 b for radiating the light beam BMb is then excited to perform the APC,and the synchronous detection is performed with its radiation beingmaintained. The radiation point 1 a for radiating the light beam BMa islast re-excited to perform the synchronous detection.

[0201] The synchronous detection is thus performed after the APCoperation, and the light beam is not eclipsed by the deflecting facet 5a also during the synchronous detection since the light beam is noteclipsed by the deflecting facet 5 a during the APC time. Thus, thesynchronous detection can be stably performed. Thereby, since asynchronous detecting unit with a convention structure can be usedwithout any change, it is possible to advantageously utilize asingle-beam scanning apparatus in a diversion way.

[0202] The number of the radiation points is two in the fifthembodiment, but the number is not limited thereto. The present inventioncan also be applied to multi-beam scanning apparatuses using three ormore radiation points. The same or more advantageous effects can also beobtained in such multi-beam scanning apparatuses using multipleradiation points such as four radiation points, eight radiation points,or sixteen radiation points.

[0203] In the fifth embodiment, the ghost light beam occurring due tothe chamfered portion 5 b is handled, but other cases can be likewisehandled. For example, even in the event that it is designed to preventinfluence of the ghost light beam reflected by the adjacent deflectingfacet due to light radiation at the time of APC, the same advantageouseffects as those of the above-discussed fifth embodiment can be obtainedin the present invention.

Sixth Embodiment

[0204]FIG. 16 is a cross-sectional view in the main scanning directionillustrating a multi-beam scanning apparatus of a sixth embodimentaccording to the present invention. In FIG. 16, like reference numeralsdesignate the same elements as those illustrated in FIG. 1.

[0205] The sixth embodiment is different from the above-discussed firstembodiment in that three radiation points are provided in a light sourceunit 81, three light beams BMa, BMb and BMc emitted from the radiationpoints 1 a, 1 b and 1 c are incident on the deflecting facet 5 a withoutintersecting each other between the light source unit 1 and thedeflecting facet 5 a, and the chamfered portion 5 b is formed at the endof the deflecting facet 5 a of the optical deflector 5 serving as therotational polygon mirror. Other construction and optical functions areapproximately the same as those of the first embodiment, and similartechnical advantages are achieved in the sixth embodiment.

[0206] In the sixth embodiment, the light source unit 1 is comprised ofa monolithic semiconductor laser array with three radiation points 1 a,1 b and 1 c. The three radiation points 1 a, 1 b and 1 c are spaced fromeach other in both the main scanning direction and the sub scanningdirection.

[0207] In the sixth embodiment, the three light beams BMa, BMb and BMcemitted from the radiation points 1 a, 1 b and 1 c are incident on thedeflecting facet 5 a without intersecting each other between the lightsource unit 1 and the deflecting facet 5 a. A portion (a BD light beam)of the light beam reflectively deflected by the deflecting facet 5 a isguided to the synchronous unit 10 through the BD mirror 9 such that thescanning start timing in the main scanning direction on the scannedsurface 8 can be determined. Further, the optical deflector 5 isprovided with the chamfered portion 5 b similar to the fifth embodiment.

[0208]FIGS. 17A to 17C illustrate manners in which the three light beamsBMa, BMb and BMc reach the deflecting facet 5 a, respectively. In FIGS.17A to 17C, like reference numerals designate the same elements as thoseillustrated in FIG. 16. In the sixth embodiment, the three light beamsBMa, BMb and BMc emitted from the light source unit 1 are incident onthe deflecting facet 5 a in such a manner that these light beamsintersect each other at a position of a point P. The three light beamsreach different locations on the deflecting facet 5 a.

[0209]FIG. 17A illustrates a manner in which the light beam BMa(indicated by a solid line) radiated for the purposes of APC of theradiation point (the third radiation point) 1 a is reflectivelydeflected by the deflecting facet 5 a. Here, the entire light beam BMais reflectively deflected by the deflecting facet 5 a, while portions ofthe other two light beams BMb and BMc stride over the chamfered portion5 b of the deflecting facet 5 a.

[0210]FIG. 17B illustrates a manner in which the light beam BMb(indicated by a dashed line) radiated for the purposes of APC of theradiation point (the second radiation point) 1 b is reflectivelydeflected by the deflecting facet 5 a. Here, the entire light beams BMaand BMb are reflectively deflected by the deflecting facet 5 a, while aportion of the light beam BMc strides over the chamfered portion 5 b ofthe deflecting facet 5 a.

[0211]FIG. 17C illustrates a manner in which the light beam BMc(indicated by an alternate long and two short dashes line) radiated forthe purposes of APC of the radiation point (the first radiation point) 1c is reflectively deflected by the deflecting facet 5 a. Here, all thethree entire light beams BMa, BMb and BMc are reflectively deflected bythe deflecting facet 5 a.

[0212] In the sixth embodiment, the three light beams BMa, BMb and BMcare incident on the deflecting facet 5 a without intersecting each otherbetween the light source unit 1 and the deflecting facet 5 a.Accordingly, the number M of light intersection is M=2×n=0 (n=0). Thedeflecting facet 5 a can be effectively used in the event that theradiation points other than the radiation point (the first radiationpoint) 1 c disposed on the most downstream side in the rotationaldirection A of the optical deflector 5 are excited in the first place.It is preferable to perform the light radiation in the order from theradiation point (the third radiation point) 1 a disposed on the mostupstream side in the rotational direction A of the optical deflector 5.

[0213] In the sixth embodiment, therefore, the radiation point (thethird radiation point) 1 a for radiating the light beam BMa disposed onthe most upstream side in the rotational direction A of the opticaldeflector 5 is firstly excited to perform the APC, the radiation point(the second radiation point) 1 b for radiating the light beam BMb isthen excited to perform the APC, and the radiation point (the firstradiation point) 1 c for radiating the light beam BMc disposed on themost downstream side in the rotational direction A of the opticaldeflector 5 is last excited to perform the APC.

[0214] Further, also during the synchronous detection, the radiationpoint 1 a for radiating the light beam BMa disposed on the most upstreamside in the rotational direction A of the optical deflector 5 is firstlyexcited to perform the synchronous detection, the radiation point 1 bfor radiating the light beam BMb is then excited to perform thesynchronous detection, and the radiation point 1 c for radiating thelight beam BMc disposed on the most downstream side in the rotationaldirection A of the optical deflector 5 is last excited to perform thesynchronous detection.

[0215] The time sequence of the radiation order is as follows. Theradiation point 1 a for radiating the light beam BMa is firstly excitedto perform the APC and then perform the synchronous detection, and isthen turned off. Next, the radiation point 1 b for radiating the lightbeam BMb is excited to perform the APC and then perform the synchronousdetection, and is then turned off. Last, the radiation point 1 c forradiating the light beam BMc is excited to perform the APC and thenperform the synchronous detection, and is then turned off.

[0216] The deflecting facet 5 a can be thus effectively used byappropriately setting the order of light radiation of the pluralradiation points. In the sixth embodiment, therefore, even when adownsized optical deflector 5 is used, the APC can be performed withoutany influence of the ghost light beam, and the synchronous detection canbe stably achieved using the same synchronous detecting unit. Therefore,the size of the multi-beam scanning apparatus can be decreased, and atthe same time the cost can be reduced.

[0217] In the sixth embodiment, similar to the above-discussed thirdembodiment, the optical path directed to the scanning optical system 7can be provided separately from the optical path directed to thesynchronous detecting unit 10.

Seventh Embodiment

[0218]FIG. 18 is a cross-sectional view in the main scanning directionillustrating a multi-beam scanning apparatus of a seventh embodimentaccording to the present invention. In FIG. 18, like reference numeralsdesignate the same elements as those illustrated in FIG. 1.

[0219] The seventh embodiment is different from the above-discussedfirst embodiment in that a unit 74 for detecting the scanning positionis provided outside the effective scanning range on the scanning finishside, an optical system (a BD lens) 73 for detecting the scanningposition is provided between the optical deflector 5 serving as therotational polygon mirror and the unit 74 for detecting the scanningposition, and the chamfered portion 5 b is formed at the end portion ofthe deflecting facet 5 a of the optical deflector 5. Other constructionand optical functions are approximately the same as those of the firstembodiment, and similar technical advantages are achieved in the seventhembodiment.

[0220] In the seventh embodiment, the unit 74 for detecting the scanningposition is comprised of a slit 74 a and a sensor 74 b such that adistance between two scanning lines can be detected using a signal fromthe sensor 74 b. The optical system (an imaging lens) 73 for detectingthe scanning position forms an image of a portion of the light beamreflectively deflected by the optical deflector 5 on the slit 74 a.

[0221] The seventh embodiment is directed to a multi-beam scanningapparatus of a type of the number M=2×n+1 (n is an integer) of lightintersection in which the light beams intersect each other only oncebetween the light source unit 1 and the deflecting facet 5 a.

[0222] Accordingly, prior to the scanning start, the radiation point(the second radiation point) 1 a disposed on the most downstream side inthe rotational direction A of the optical deflector 5 is excited in thefirst place, and the radiation point (the first radiation point) 1 bdisposed on the upstream side in the rotational direction A of theoptical deflector 5 is then excited.

[0223] Specifically, the radiation point 1 a for radiating the lightbeam BMa is excited to perform the APC, and is then turned off. Next,the radiation point 1 b for radiating the light beam BMb disposed on theupstream side in the rotational direction A of the optical deflector 5is excited to perform the APC and then perform the synchronous detectionwith its radiation being continued. Then, the radiation point 1 a isre-excited to perform the synchronous detection.

[0224] Further, also after the scanning is finished, the radiation point(the third radiation point) 1 a disposed on the downstream side in therotational direction A of the optical deflector 5 is firstly excited,and the radiation point 1 b (the first radiation point) disposed on theupstream side in the rotational direction A of the optical deflector 5is then excited.

[0225] The deflecting can be thus effectively used by appropriatelysetting the light radiation order in the event that the plural lightbeams are radiated immediately prior to the start of scanning, or in theevent that the plural light beams are radiated immediately subsequent tothe finish of scanning. In the seventh embodiment, therefore, increasein the size of the optical deflector can be prevented.

[0226] Further, in the event that the plural light beams are radiatedimmediately before and after the effective scanning range on the scannedsurface 8 as in the seventh embodiment, advantageous effects of thepresent invention can be more effectively obtained.

[0227] In the seventh embodiment, though description is made to themulti-beam scanning optical system of the type in which the light beamsintersect each other once between the light source unit 1 and thedeflecting facet 5 a, and the light intersection number (M=2×n+1) is anodd number, the present invention can also be applied to a multi-beamscanning optical system of a type in which the light intersection number(M=2×n) is zero, or an even number. In this case, the light radiationorder needs only to be reversed. In other words, in the event that theplural light beams are radiated immediately prior to the start ofscanning, or in the event that the plural light beams are radiatedimmediately subsequent to the finish of scanning, it is preferable tofirstly excite the radiation point disposed on the upstream side in therotational direction A of the optical deflector 5. More preferably, theradiation point disposed on the upstream side in the rotationaldirection A of the optical deflector 5 is excited in the first place,and thereafter the radiation point disposed on the more downstream sideis excited in the order.

[0228] In the first to seventh embodiments, description has been made toapparatuses in which the radiation points disposed with being spacedfrom each other in both the main scanning direction and the sub scanningdirection are composed of two or more monolithic semiconductor lasers ofedge emitting types, but the present invention can also be applied to anapparatus in which radiation points are comprised of three or moresemiconductor lasers of surface emitting types. For example, theapparatus can use a surface emitting semiconductor laser in which fouror more radiation points are arranged in a two-dimensional array.

[0229] Further, since the present invention aims at solving the problemthat is likely to occur in apparatuses in which two or more light beamsare incident on the deflecting facet of a deflecting unit (for example,a rotational polygon mirror) at different incident angles, the presentinvention can also be applied to apparatuses in which two or moremonolithic semiconductor lasers each including at least one radiationpoint is used, composition of the two or more light beams is executed bya beam composite system (a polarization beam splitter, or the like), andthe two or more light beams are incident on the deflecting facet of adeflecting unit (for example, a rotational polygon mirror) at differentincident angles.

Image Forming Apparatus

[0230]FIG. 19 is a cross-sectional view of a main portion along the subscanning direction illustrating an embodiment of an image formingapparatus according to the present invention. In FIG. 19, referencenumeral 104 designates an image forming apparatus. This image formingapparatus 104 accepts input of code data Dc from an external device 117,such as a personal computer or the like. This code data Dc is convertedinto image data (dot data) Di by a printer controller 111 in theapparatus 104. This image data Di is supplied to A multi-beam opticalscanning apparatus 100 having the structure as described in either ofthe above embodiments. This optical scanning apparatus 100 outputs twoor more light beams 103 modulated according to the image data Di, andthese light beams 103 scan a photosensitive surface of a photosensitivedrum 101 in the main scanning direction.

[0231] The photosensitive drum 101 serving as an electrostatic latentimage carrier (a photosensitive body) is rotated in a clockwisedirection by a motor 115. With the rotation thereof, the photosensitivesurface of the photosensitive drum 101 moves in the sub scanningdirection perpendicular to the main scanning direction, relative to thetwo or more light beams 103. Above the photosensitive drum 101, acharging roller 102 for uniformly charging the surface of thephotosensitive drum 101 is disposed so as to contact the surface. And,the surface of the photosensitive drum 101 charged by the chargingroller 102 is exposed to the light beams 103 scanned by the opticalscanning apparatus 100.

[0232] As described previously, the light beams 103 are modulated basedon the image data Di, and electrostatic latent images are formed on thesurface of the photosensitive drum 101 under irradiation with the lightbeams 103. These electrostatic latent images are developed into tonerimages by a developing unit 107 disposed so as to contact thephotosensitive drum 101 downstream in the rotating direction of thephotosensitive drum 101 from the irradiation position of the light beams103.

[0233] The toner image developed by the developing unit 107 istransferred onto a sheet 112 which is a transfer material, by a transferroller 108 disposed opposed to the photosensitive drum 101 below thephotosensitive drum 101. Sheets 112 are stored in a sheet cassette 109in front of the photosensitive drum 101 (on a right side of FIG. 19),but sheet feed can also be performed by hand feeding. A sheet feedroller 110 is disposed at an end of the sheet cassette 109, and feedseach sheet 112 in the sheet cassette 109 into the conveyance path.

[0234] The sheet 112, onto which the unfixed toner image is transferredas described above, is further transferred to a fixing unit locatedbehind the photosensitive drum 101 (i.e., on a left side of FIG. 19).The fixing unit is composed of a fixing roller 113 having a fixingheater (not illustrated) inside and a pressing roller 114 disposed inpressure contact with the fixing roller 113, and heats the sheet 112while pressing the sheet 112 thus conveyed from the transfer part, in anip portion between the fixing roller 113 and the pressing roller 114,to fix the unfixed toner image on the sheet 112. Sheet discharge rollers116 are further disposed behind the fixing roller 113 to discharge thefixed sheet 112 to the outside of the image forming apparatus 104.

[0235] Although not illustrated in FIG. 19, the print controller 111also performs control of each section in the image forming apparatus,including the motor 115, and control of a polygon motor, etc., in Amulti-beam optical scanning apparatus 104 described above, in additionto the conversion of data described above.

Color Image Forming Apparatus

[0236]FIG. 20 is a schematic view illustrating a main portion of a colorimage forming apparatus of the present invention. This embodiment isdirected to a color image forming apparatus of a tandem type in whichfour scanning apparatuses using plural light beams are arranged in aparallel manner, and image information is recorded on eachphotosensitive drum serving as an image carrier. In FIG. 20, referencenumeral 60 represents a color image forming apparatus. Referencenumerals 11, 12, 13 and 14 represent optical scanning apparatuses asdescribed in the above embodiments of the scanning apparatuses,respectively. Reference numerals 21, 22, 23 and 24 representphotosensitive drums serving as image carrier, respectively. Referencenumerals 31, 32, 33 and 34 represent developing units, respectively.Reference numeral 51 represents a conveyance belt.

[0237] In FIG. 20, the color image forming apparatus 60 accepts input ofcolor signals of R (red), G (green) and B (blue) from an external device52 such as a personal computer. Those color signals are converted intoimage data (dot data) of C (cyan), M (magenta), Y (yellow), and Bk(black) by a printer controller 53 in the apparatus. The image data issupplied to the optical scanning apparatuses 11, 12, 13 and 14. Each ofthose optical scanning apparatuses 11, 12, 13 and 14 outputs a lightbeam 41, 42, 43 or 44 modulated according to each image data, and theselight beams scan photosensitive surfaces of photosensitive drums 21, 22,23 and 24 in the main scanning direction, respectively.

[0238] In the color image forming apparatus of this embodiment, thereare provided four light scanning apparatuses 11, 12, 13 and 14corresponding to colors of C (cyan), M (magenta), Y (yellow), and Bk(black), respectively, and these scanning apparatuses record imagesignals (image data) on the photosensitive drums 21, 22, 23 and 24 in aparallel manner, respectively, to speedily print a color image.

[0239] In the color image forming apparatus of this embodiment, latentimages of colors are formed on corresponding photosensitive drums 21,22, 23 and 24 by the four optical scanning apparatuses 11, 12, 13 and 14using two or more light beams based on the image data, respectively.After that, the latent images are multi-transferred onto a recordingmaterial, and a full-color picture is thus formed.

[0240] As the external device 52, a color image reading apparatusprovided with a CCD sensor can be used, for example. In this case, thiscolor image reading apparatus and the color image forming apparatus 60constitute a color digital copying apparatus.

[0241] The present invention can provide more preeminent technicaladvantages in the tandem-type color image forming apparatus of FIG. 20.The reason therefor is as follows. In the tandem-type color imageforming apparatus, plural (two or more) light beams are incident ondifferent photosensitive drums 21, 22, 23 and 24, respectively, imagesare formed on different scanned surfaces for respective colors, andthereafter the respective color images are superimposed on a sheet, sothat color shift (registration shift) due to the shift in scanning startpositions of plural light beams incident on the same scanned surface isa serious problem.

[0242] According to the present invention, as described in theforegoing, the radiation point for radiating the light beam reaching theplace nearest the center of the deflecting facet of the deflecting unitis excited in the first place. Accordingly, the deflecting facet can beeffectively used, and the optical deflecting unit can be downsized.Subsequently, the entire apparatus can be downsized, and a multi-beamscanning apparatus capable of readily constructing structures of thedeflecting unit, the motor, and the like can be provided.

[0243] While the present invention has been described with reference towhat are presently considered to be the preferred embodiments, it is tobe understood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodications and equivalent structures and functions.

What is claimed is:
 1. A multi-beam optical scanning apparatus comprising: light source means including a plurality of radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting a plurality of light beams radiated from said plurality of radiation points toward a surface to be scanned; wherein where a first radiation point is a radiation point for radiating the light beam, out of the plurality of light beams emitted from said plurality of radiation points, which reaches the farthest location from a center of a deflecting facet of said deflecting means in the main scanning direction, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on an upstream side in a rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 2. A multi-beam optical scanning apparatus according to claim 1, wherein the light beam of said radiation point for radiating the light beam reaching a location nearest a center of the deflecting facet of said deflecting means is radiated in the first place, out of the plurality of light beams radiated by said light source means.
 3. A multi-beam optical scanning apparatus according to claim 2, wherein the light beam of said radiation point for radiating the light beam reaching a location nearer the center of the deflecting facet of said deflecting means is radiated in the order from the nearest location, out of the plurality of light beams radiated by said light source means.
 4. A multi-beam optical scanning apparatus comprising: light source means including a plurality of radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting a plurality of light beams radiated from said plurality of radiation points toward a surface to be scanned, the plurality of light beams radiated from said plurality of radiation points intersecting each other M times (M=2n+1; n is an integer) between said light source means and said deflecting means; wherein where a first radiation point is a radiation point disposed on a most upstream side in a rotational direction of said deflecting means, out of said plurality of radiation points, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on the upstream side in the rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 5. A multi-beam optical scanning apparatus according to claim 4, wherein the light beam of said radiation point disposed on a most downstream side in the rotational direction of said deflecting means is radiated in the first place.
 6. A multi-beam optical scanning apparatus according to claim 5, wherein the light beam of said radiation point disposed on the more downstream side in the rotational direction of said deflecting means is radiated in the order from the most downstream side.
 7. A multi-beam optical scanning apparatus according to claim 4, wherein the radiation amount of the light beam is adjusted by radiating the light beam from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 8. A multi-beam optical scanning apparatus according to claim 4, further comprising a scanning optical system for forming images of the plurality of light beams deflected by said deflecting means on the surface to be scanned; and synchronous detecting means for detecting writing start timings on the surface to be scanned by receiving the plurality of light beams deflected by said deflecting means; and wherein synchronous detection is performed by radiating the light beam directed to said synchronous detecting means from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 9. A multi-beam optical scanning apparatus according to claim 4, wherein a chamfered portion is formed at an edge of a deflecting facet of said deflecting means.
 10. A multi-beam optical scanning apparatus according to claim 4, wherein where a third radiation point is another radiation point other than said first radiation point disposed on the most upstream side in the rotational direction of said deflecting means, the light beam of said third radiation point is radiated in the first place in a downstream-side external angular range subsequent to the effective scanning range on the surface to be scanned.
 11. A multi-beam optical scanning apparatus comprising: light source means including a plurality of radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting a plurality of light beams radiated from said plurality of radiation points toward a surface to be scanned, the plurality of light beams radiated from said plurality of radiation points intersecting each other N times (N=2n; n is an integer) between said light source means and said deflecting means; wherein where a first radiation point is a radiation point disposed on a most downstream side in a rotational direction of said deflecting means, out of said plurality of radiation points, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on the upstream side in the rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 12. A multi-beam optical scanning apparatus according to claim 11, wherein the light beam of said radiation point disposed on the most upstream side in the rotational direction of said deflecting means is radiated in the first place.
 13. A multi-beam optical scanning apparatus according to claim 12, wherein the light beam of said radiation point disposed on the more upstream side in the rotational direction of said deflecting means is radiated in the order from the most upstream side.
 14. A multi-beam optical scanning apparatus according to claim 11, wherein the radiation amount of the light beam is adjusted by radiating the light beam from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 15. A multi-beam optical scanning apparatus according to claim 11, further comprising a scanning optical system for forming images of the plurality of light beams deflected by said deflecting means on the surface to be scanned; and synchronous detecting means for detecting writing start timings on the surface to be scanned by receiving the plurality of light beams deflected by said deflecting means; and wherein synchronous detection is performed by radiating the light beam directed to said synchronous detecting means from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 16. A multi-beam optical scanning apparatus according to claim 11, wherein a chamfered portion is formed at an edge of a deflecting facet of said deflecting means.
 17. A multi-beam optical scanning apparatus according to claim 11, wherein where a third radiation point is another radiation point other than said first radiation point disposed on the most downstream side in the rotational direction of said deflecting means, the light beam of said third radiation point is radiated in the first place in a downstream-side external angular range subsequent to the effective scanning range on the surface to be scanned.
 18. A multi-beam optical scanning apparatus comprising: light source means including a plurality of radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting a plurality of light beams radiated from said plurality of radiation points toward a surface to be scanned; wherein the light beam of said radiation point for radiating the light beam firstly incident on a deflecting facet of said deflecting means in the main scanning direction is radiated prior to the light beam from the other radiation point.
 19. A multi-beam optical scanning apparatus according to claim 18, further comprising a scanning optical system for forming images of the plurality of light beams deflected by said deflecting means on the surface to be scanned; and synchronous detecting means for detecting writing start timings on the surface to be scanned by receiving the plurality of light beams deflected by said deflecting means; and wherein synchronous detection is performed by radiating the light beam directed to said synchronous detecting means from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 20. A multi-beam optical scanning apparatus according to any one of claims 1, 4, 11 and 18, wherein said light source means is comprised of a monolithic semiconductor laser.
 21. An image forming apparatus comprising: a multi-beam optical scanning apparatus recited in any one of claims 1, 4, 11 and 18; an image bearing member placed at the surface to be scanned; developing means for developing an electrostatic latent image, which is formed on said image bearing member by the light beam scanned by said multi-beam optical scanning apparatus, as a toner image; transferring means for transferring the developed toner image onto a transferring material; and fixing means for fixing the transferred toner image on the transferring material.
 22. An image forming apparatus comprising: a multi-beam optical scanning apparatus recited in claim 21; and a printer controller for converting code data input from an external apparatus into an image signal to supply the image signal to said multi-beam optical scanning apparatus.
 23. A color image forming apparatus comprising: a plurality of multi-beam optical scanning apparatuses each of which includes a multi-beam optical scanning apparatus recited in any one of claims 1, 4, 11 and 18; and a plurality of image bearing members each of which is placed at the surface to be scanned of said each multi-beam optical scanning apparatus, and which form images of different colors, respectively.
 24. A color image forming apparatus comprising: a multi-beam optical scanning apparatus recited in claim 23; and a printer controller for converting code data input from an external apparatus into an image signal to supply the image signal to said multi-beam optical scanning apparatus.
 25. A multi-beam optical scanning apparatus comprising: light source means including at least three radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting at least three light beams radiated from said at least three radiation points toward a surface to be scanned; wherein where a first radiation point is a radiation point for radiating the light beam, out of the at least three light beams emitted from said at least three radiation points, which reaches the farthest location from a center of a deflecting facet of said deflecting means in the main scanning direction, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on an upstream side in a rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 26. A multi-beam optical scanning apparatus according to claim 25, wherein the light beam of said radiation point for radiating the light beam reaching a location nearest a center of the deflecting facet of said deflecting means is radiated in the first place, out of the at least three light beams radiated by said light source means.
 27. A multi-beam optical scanning apparatus according to claim 26, wherein the light beam of said radiation point for radiating the light beam reaching a location nearer the center of the deflecting facet of said deflecting means is radiated in the order from the nearest location, out of the at least three light beams radiated by said light source means.
 28. A multi-beam optical scanning apparatus comprising: light source means including at least three radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting at least three light beams radiated from said at least three radiation points toward a surface to be scanned, the at least three light beams radiated from said at least three radiation points intersecting each other M times (M=2n+1; n is an integer) between said light source means and said deflecting means; wherein where a first radiation point is a radiation point disposed on a most upstream side in a rotational direction of said deflecting means, out of said at least three radiation points, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on the upstream side in the rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 29. A multi-beam optical scanning apparatus according to claim 28, wherein the light beam of said radiation point disposed on a most downstream side in the rotational direction of said deflecting means is radiated in the first place.
 30. A multi-beam optical scanning apparatus according to claim 29, wherein the light beam of said radiation point disposed on the more downstream side in the rotational direction of said deflecting means is radiated in the order from the most downstream side.
 31. A multi-beam optical scanning apparatus according to claim 28, wherein the radiation amount of the light beam is adjusted by radiating the light beam from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 32. A multi-beam optical scanning apparatus according to claim 28, further comprising a scanning optical system for forming images of the at least light beams deflected by said deflecting means on the surface to be scanned; and synchronous detecting means for detecting writing start timings on the surface to be scanned by receiving the at least three light beams deflected by said deflecting means; and wherein synchronous detection is performed by radiating the light beam directed to said synchronous detecting means from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 33. A multi-beam optical scanning apparatus according to claim 28, wherein a chamfered portion is formed at an edge of a deflecting facet of said deflecting means.
 34. A multi-beam optical scanning apparatus according to claim 28, wherein where a third radiation point is another radiation point other than said first radiation point disposed on the most upstream side in the rotational direction of said deflecting means, the light beam of said third radiation point is radiated in the first place in a downstream-side external angular range subsequent to the effective scanning range on the surface to be scanned.
 35. A multi-beam optical scanning apparatus comprising: light source means including at least three radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting at least three light beams radiated from said at least three radiation points toward a surface to be scanned, the at least three light beams radiated from said at least three radiation points intersecting each other N times (N=2n; n is an integer) between said light source means and said deflecting means; wherein where a first radiation point is a radiation point disposed on a most downstream side in a rotational direction of said deflecting means, out of said at least three radiation points, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on the upstream side in the rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 36. A multi-beam optical scanning apparatus according to claim 35, wherein the light beam of said radiation point disposed on the most upstream side in the rotational direction of said deflecting means is radiated in the first place.
 37. A multi-beam optical scanning apparatus according to claim 36, wherein the light beam of said radiation point disposed on the more upstream side in the rotational direction of said deflecting means is radiated in the order from the most upstream side.
 38. A multi-beam optical scanning apparatus according to claim 35, wherein the radiation amount of the light beam is adjusted by radiating the light beam from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 39. A multi-beam optical scanning apparatus according to claim 35, further comprising a scanning optical system for forming images of the at least three light beams deflected by said deflecting means on the surface to be scanned; and synchronous detecting means for detecting writing start timings on the surface to be scanned by receiving the at least three light beams deflected by said deflecting means; and wherein synchronous detection is performed by radiating the light beam directed to said synchronous detecting means from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 40. A multi-beam optical scanning apparatus according to claim 35, wherein a chamfered portion is formed at an edge of a deflecting facet of said deflecting means.
 41. A multi-beam optical scanning apparatus according to claim 35, wherein where a third radiation point is another radiation point other than said first radiation point disposed on the most downstream side in the rotational direction of said deflecting means, the light beam of said third radiation point is radiated in the first place in a downstream-side external angular range subsequent to the effective scanning range on the surface to be scanned.
 42. A multi-beam optical scanning apparatus comprising: light source means including at least three radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting at least three light beams radiated from said at least three radiation points toward a surface to be scanned; wherein the light beam of said radiation point for radiating the light beam firstly incident on a deflecting facet of said deflecting means in the main scanning direction is radiated prior to the light beam from the other radiation point.
 43. A multi-beam optical scanning apparatus according to claim 42, further comprising a scanning optical system for forming images of the at least three light beams deflected by said deflecting means on the surface to be scanned; and synchronous detecting means for detecting writing start timings on the surface to be scanned by receiving the at least three light beams deflected by said deflecting means; and wherein synchronous detection is performed by radiating the light beam directed to said synchronous detecting means from said radiation point of said light source means in the upstream-side external angular range prior to the effective scanning range on the surface to be scanned.
 44. A multi-beam optical scanning apparatus according to any one of claims 25, 28, 35 and 42, wherein said light source means is comprised of a monolithic semiconductor laser.
 45. An image forming apparatus comprising: a multi-beam optical scanning apparatus recited in any one of claims 25, 28, 35 and 42; an image bearing member placed at the surface to be scanned; developing means for developing an electrostatic latent image, which is formed on said image bearing member by the light beam scanned by said multi-beam optical scanning apparatus, as a toner image; transferring means for transferring the developed toner image onto a transferring material; and fixing means for fixing the transferred toner image on the transferring material.
 46. An image forming apparatus comprising: a multi-beam optical scanning apparatus recited in claim 45; and a printer controller for converting code data input from an external apparatus into an image signal to supply the image signal to said multi-beam optical scanning apparatus.
 47. A color image forming apparatus comprising: a plurality of multi-beam optical scanning apparatuses each of which includes a multi-beam optical scanning apparatus recited in any one of claims 25, 28, 35 and 42; and a plurality of image bearing members each of which is placed at the surface to be scanned of said each multi-beam optical scanning apparatus, and which form images of different colors, respectively.
 48. A color image forming apparatus comprising: a multi-beam optical scanning apparatus recited in claim 47; and a printer controller for converting code data input from an external apparatus into an image signal to supply the image signal to said multi-beam optical scanning apparatus.
 49. A multi-beam optical scanning apparatus comprising: light source means including a plurality of radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting a plurality of light beams radiated from said plurality of radiation points toward a surface to be scanned; wherein where a first radiation point is a radiation point for radiating the light beam, out of the plurality of light beams emitted from said plurality of radiation points, which reaches the farthest location from a center of a deflecting facet of said deflecting means in the main scanning direction, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on an upstream side in a rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, a width of the deflecting facet in a main scanning section is set to such a magnitude that the light beam reaching the location most spaced from the center of the deflecting facet at an end portion of the deflecting facet is eclipsed in the event that the light beam from said first radiation point for radiating the light beam reaching the location most spaced from the center of the deflecting facet is radiated prior to the light beam from said second radiation point in the upstream-side external angular range, and control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 50. A multi-beam optical scanning apparatus comprising: light source means including a plurality of radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting a plurality of light beams radiated from said plurality of radiation points toward a surface to be scanned; wherein a width of the deflecting facet in a main scanning section is set to such a magnitude that the light beam last incident on an end portion of the deflecting facet is eclipsed in the event that the light beam from said radiation point for radiating the light beam last incident on the deflecting facet of said deflecting means is radiated prior to the light beam from the other radiation point, and the light beam of said radiation point for radiating the light beam firstly incident on the deflecting facet of said deflecting means in the main scanning direction is radiated prior to the light beam from the other radiation point.
 51. A multi-beam optical scanning apparatus comprising: light source means including at least three radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting at least three light beams radiated from said at least three radiation points toward a surface to be scanned; wherein where a first radiation point is a radiation point for radiating the light beam, out of the at least three light beams emitted from said at least three radiation points, which reaches the farthest location from a center of a deflecting facet of said deflecting means in the main scanning direction, a second radiation point is a radiation point for radiating another light beam, and an upstream-side external angular range is a range which lies in an angular range over which the light beam can be deflected by said deflecting means, and which exists on an upstream side in a rotational direction of said deflecting means relative to an effective scanning angular range at the time when the light beam is deflected toward an effective scanning range on the surface to be scanned, a width of the deflecting facet in a main scanning section is set to such a magnitude that the light beam reaching the farthest location from a center of the deflecting facet at an end portion of the deflecting facet is eclipsed in the event that the light beam from said first radiation point for radiating the light beam reaching the location most spaced from the center of the deflecting facet is radiated prior to the light beam from said second radiation point in the upstream-side external angular range, and control is performed such that the light beam from said second radiation point can be radiated prior to the light beam from said first radiation point in the upstream-side external angular range.
 52. A multi-beam optical scanning apparatus comprising: light source means including at least three radiation points disposed with being spaced from each other in a main scanning direction; and deflecting means for deflecting at least three light beams radiated from said at least three radiation points toward a surface to be scanned; wherein a width of a deflecting facet of said deflecting means in a main scanning section is set to such a magnitude that the light beam last reaching an end portion of the deflecting facet is eclipsed in the event that the light beam from said radiation point for radiating the light beam last incident on the deflecting facet of said deflecting means is radiated prior to the light beam from the other radiation point, and the light beam from said radiation point for radiating the light beam firstly incident on the deflecting facet of said deflecting means in the main scanning direction is radiated prior to the light beam from the other radiation point.
 53. An image forming apparatus comprising: a multi-beam optical scanning apparatus recited in any one of claims 49 to 52; an image bearing member placed at the surface to be scanned; developing means for developing an electrostatic latent image, which is formed on said image bearing member by the light beam scanned by said multi-beam optical scanning apparatus, as a toner image; transferring means for transferring the developed toner image onto a transferring material; and fixing means for fixing the transferred toner image on the transferring material.
 54. An image forming apparatus comprising: a multi-beam optical scanning apparatus recited in claim 53; and a printer controller for converting code data input from an external apparatus into an image signal to supply the image signal to said multi-beam optical scanning apparatus.
 55. A color image forming apparatus comprising: a plurality of multi-beam optical scanning apparatuses each of which includes a multi-beam optical scanning apparatus recited in any one of claims 49 to 52; and a plurality of image bearing members each of which is placed at the surface to be scanned of said each multi-beam optical scanning apparatus, and which form images of different colors, respectively.
 56. A color image forming apparatus comprising: a multi-beam optical scanning apparatus recited in claim 55; and a printer controller for converting code data input from an external apparatus into an image signal to supply the image signal to said multi-beam optical scanning apparatus. 